Ukrainian tanks are transported from their base in Perevalne, Crimea, on Wednesday, March 26. After Russian troops seized most of Ukraine's bases in Crimea, interim Ukrainian President Oleksandr Turchynov ordered the withdrawal of armed forces from the peninsula, citing Russian threats to the lives of military staff and their families.
Simferopol, Ukraine (CNN) -- Hostilities intensified in Crimea on Saturday as Ukrainian officials accused pro-Russian forces of armed aggression and President Barack Obama rounded up world leaders to demand Russia "de-escalate the situation."
Obama called British, French and Italian leaders and hosted a conference call with the presidents of Lithuania, Latvia and Estonia, the White House said.
"All of the leaders agreed on the need for Russia to pull its military forces back to their bases, allow for the deployment of international observers and human rights monitors to the Crimean peninsula, and agree quickly on the formation of a contact group that could lead to direct dialogue between Ukraine and Russia to de-escalate the situation and restore Ukraine's sovereignty and territorial integrity," a White House statement said.
They also rejected a proposed referendum in Crimea on whether it should rejoin Russia "as a violation of Ukraine's constitution," the White House said.
"The leaders made clear that Russia's continued violation of international law will isolate it from the international community," the White House said.
Secretary of State John Kerry phoned Russian Foreign Minister Sergey Lavrov on Saturday and issued a diplomatic ultimatum, according to a senior U.S. administration official.
"He made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia would close any available space for diplomacy, and he urged utmost restraint," the official said.
Kerry said the United States is ready to work with allies to facilitate a Ukraine-Russia dialogue, the official said.
French President Francois Hollande said he and Obama "discussed the need for Russia and Ukraine to find a peaceful exit from the crisis and to fully restore the sovereignty and territorial integrity of Ukraine," Hollande's office said.
Tense in Crimea
Meanwhile, Ukrainian officials charged that pro-Russian forces comprised of 100 armed men reportedly took control of a military office in the Crimean regional capital, Simferopol.
The men, equipped with automatic weapons, say they belong to the Crimean self-defense forces, Vladislav Seleznyov, the head of the Ministry of Defense media office, said on his Facebook page.
A CNN team that visited the scene said it appeared calm. Armed, masked men were at the entrance, and Russian flags were being painted on the gates. Those questioned declined to say what was happening inside.
Amid signs that the tense standoff of the past week is growing more volatile, Russian troops also stormed a Crimean border control point at Schelkino, near Kerch, early Saturday, seizing the armory and driving the officers' families from their living quarters, Ukraine's border service said.
Meanwhile, a light plane belonging to Ukraine's State Border Protection Service was fired upon Saturday afternoon while flying over the Crimean border from Armyansk, the service said.
"The aircraft crew recorded shooting aimed at the plane. The extremists opened fire with automatic weapons. The pilots made a sudden maneuver, descended to the minimal height and, with accelerated speed, changed the course," the border guard service said.
The plane landed at its base, with no damage and no injuries to the crew, the service said.
It blamed "Russian armed aggressors" for targeting the plane, which has no weapons.
Armed men have now refused for three consecutive days to allow military observers from the Organization for Security and Co-operation in Europe, or OSCE, to enter the Crimea region.
The observers are returning to the southern Ukrainian city of Kherson to plan their next steps after being denied entry at the Armyansk crossing point Saturday, the OSCE said.
The OSCE said shots were fired in the air when a group of people wearing balaclavas approached the checkpoint ahead of its party.
A convoy of military vehicles, believed to be carrying Russian soldiers, traveled through Simferopol on Saturday, heading toward the border post at Armyansk, a spokesman for the Ukrainian Ministry of Defense told CNN.
Armyansk is one of the key main access points between southern Ukraine and Crimea, and the Ministry of Defense is trying to find out if the convoy will stop there, said Vladislav Seleznev, the Crimean press secretary for the Ministry of Defense.
Poland evacuates consulate
The crisis in Crimea began about a week ago, when pro-Russian troops quietly took effective control of the region.
Since then, tensions have flared between Moscow and Kiev over the Black Sea peninsula, while the world's diplomats have urged that conflict be avoided.
Earlier Saturday, Moscow accused the OSCE, a regional advisory group, of hypocrisy.
First, condemn violence by demonstrators in Kiev that led to the ouster of Ukraine's former President Viktor Yanukovych, the foreign ministry said, according to state-run news agency RIA Novosti. Then talk about Crimea.
"Some OSCE members and executive bodies have acted in the worst traditions of double standards while dealing with the situation in Ukraine," the ministry reportedly said.
Amid mounting tensions, Poland has decided to pull staff from its consulate in Sevastopol, the Crimean port city where Russia has a large naval base.
Polish Foreign Minister Radoslaw Sikorski tweeted, "Because of continuing disturbances by Russian forces there, we have reluctantly evacuated our consulate."
Russian demands
Earlier Saturday, Lavrov insisted Russia was not militarily involved in the standoff in Crimea.
"We are ready to continue dialogue, with the understanding that the dialogue will be honest and a partnership, and without attempts to cast us as a party to the conflict -- which is what a few of our partners are trying to do now. This crisis was not created by us," he said.
That dialogue might not include Ukraine's new interim government, whom Lavrov criticized.
"The current government is dependent on the radical nationalists who seized the power," he said. "Even our Western partners know what they are like. They visit Ukraine often. But they try to hide the facts."
The aim of dialogue would be the implementation of a deal agreed upon on February 21, Lavrov said, when Yanukovych was still in power. This envisaged fresh elections, constitutional reform and the disbanding of illegal armed groups.
Ukrainian authorities and Western powers have said there are Russian troops on the ground in Crimea, despite the Kremlin's denials.
The interim government was voted in by a large majority in Ukraine's parliament, including members of Yanukovych's own Party of Regions, after he fled the country for Russia following bloody street protests. Protesters were angry over his rebuff of a trade deal with the European Union in favor of one with Russia.
Ukraine: Let observers into Crimea
Ukrainian Acting Foreign Minister Andrii Deshchytsia urged Moscow on Saturday to pursue diplomacy, not violence.
"We cannot afford to let anybody die in this conflict," he said.
Ukraine will continue down a diplomatic path, he said, and will not respond to Russian "provocations."
He urged Russia not to block international organizations like the OSCE and United Nations from sending observers into Crimea.
Russia says the armed men who have taken control of key sites in Crimea are local self-defense forces.
Crimea, a self-governing region in southern Ukraine with an ethnic Russian majority and strong cultural ties to Russia, has become the epicenter of a battle for influence between Moscow, Kiev and the West since Yanukovych was pushed out of office.
Complete coverage on the Ukraine crisis
Russian fears
Anton Fedyashin, executive director of the Initiative for Russian Culture and a professor of history at American University, told CNN that Russia had nothing to gain from the current tensions in Ukraine.
"There is no advantage for Russia to stoke this instability," he said. "A lot of it is instability that is born of the great cultural, ethnic and now political divisions that are on the ground."
While most Ukrainians do not want to be allied solely with either Russia or Europe, he said, Moscow is definitely concerned by Kiev's turn toward the West.
"The great Russian fear right now is that Ukraine, in the guise of being incorporated economically into Europe, is slowly incorporated into the NATO alliance," he said.
"Ultimately, of course, Russians are concerned about their Black Sea Fleet in Sevastopol. This explains their very harsh and highly risky reaction, and the presence of what seem to be Russian forces in the Crimea."
Russian speakers make up about 60% of Crimea's population of more than 2 million, but around a quarter are Ukrainian and 12% are Crimean Tatar, a predominantly Muslim minority. Neither of the latter two groups would welcome a switch to Russian control.
President Obama has proposed a potential solution to Putin that would include direct talks between Kiev and Moscow and the replacement of Russian forces with international monitors to protect the rights of ethnic Russians.
International Women's Day
Women in Ukraine are typically celebratory during International Women's Day, held Saturday, but not this year.
The military tensions have cast a pall over the day, said Maia Mikhaluk, a CNN iReporter in Kiev.
"Usually it's a celebration that symbolizes the beginning of spring. But right now it's hard to get into any kind for festive mood," Mikhaluk said. "The most common wish when people congratulate women with Women's Day today is a wish for peace in our country."
In Simferopol, women voiced opposition to the pro-Russian Crimean government and advocated the peninsula remaining as part of Ukraine.
Asset freezes, visa bans
The West has also offered financial support to the fledgling government in Kiev.
Ukraine's new government and the EU have agreed to revive a trade deal and an aid package that could bring $15 billion to Ukraine.
Such aid is desperately needed.
Russian energy giant Gazprom has not received any payment from Ukraine in February -- and cannot supply natural gas for free, the company's CEO Alexey Miller said, according to Russian state news agency Itar-Tass.
If Gazprom cut off gas supplies to Ukraine, as it did during a dispute in 2009, then European nations could also suffer shortages, since major gas pipelines pass through Ukraine on their way from Russia to Europe.
In Crimea: 'We will protect our land from Western extremists'
Ukraine PM: Crimea 'was, is and will be an integral part of Ukraine'
CNN's Matthew Chance reported from Simferopol, and Laura Smith-Spark wrote from London. CNN's Anna Coren in Simferopol, Kellie Morgan in Kherson, Tim Schwarz in Kiev, Alla Eshchenko in Moscow, Bharati Naik, Chelsea J. Carter, Yon Pomrenze and Michael Martinez also contributed to this report.
Ukrainian marines wave as they leave a base in Feodosia, Crimea, on Tuesday, March 25.
Russian sailors stand on the deck of the corvette ship Suzdalets in the bay of Sevastopol, Crimea, on March 25.
Pro-Russian militia members remove a local resident as Russian troops assault the Belbek air base, outside Sevastopol, on Saturday, March 22. Following its annexation of Crimea, Russian forces have consolidated their control of the region.
Soldiers in unmarked uniforms sit atop an armored personnel carrier at the gate of the Belbek air base on March 22.
A Russian sailor holds the Russian Navy's St. Andrew's flag while standing on the bow of the surrendered Ukrainian submarine Zaporozhye on March 22 in Sevastopol.
Russian President Vladimir Putin signs the final decree completing the annexation of Crimea on Friday, March 21, as Upper House Speaker Valentina Matviyenko, left, and State Duma Speaker Sergei Naryshkin watch.
A Ukrainian serviceman leaves a Ukrainian military unit that Russian soldiers took control of in Perevalne on March 21.
Ukrainian border guards run during training at a military camp in Alekseyevka, Ukraine, on March 21.
Russian soldiers patrol the area surrounding a Ukrainian military unit in Perevalne on Thursday, March 20.
Pro-Russian protesters remove the gate to the Ukrainian navy headquarters as Russian troops stand guard in Sevastopol on Wednesday, March 19.
Pro-Russian forces walk inside the Ukrainian navy headquarters in Sevastopol on March 19.
A member of pro-Russian forces takes down a Ukrainian flag at the Ukrainian navy headquarters in Sevastopol on March 19.
Alexander Vitko, chief of the Russian Black Sea Fleet, leaves the Ukrainian navy headquarters in Sevastopol after pro-Russian forces took it over on March 19.
A Russian flag waves as workers install a new sign on a parliament building in Simferopol, Crimea's capital, on March 19.
Russian military personnel surround a Ukrainian military base in Perevalne on March 19.
Nameplates on the front of the Crimean parliament building get removed Tuesday, March 18, in Simferopol.
From left, Crimean Prime Minister Sergey Aksyonov; Vladimir Konstantinov, speaker of the Crimean parliament; Russian President Vladimir Putin; and Alexei Chaly, the new de facto mayor of Sevastopol, join hands in Moscow on March 18 after signing a treaty to make Crimea part of Russia.
Demonstrators hold a Crimean flag at Lenin Square in Simferopol on March 18.
Ukrainian soldiers stand guard at a checkpoint near Strilkove, Ukraine, close to Crimea on Monday, March 17.
Former boxer and Ukrainian politician Vitali Klitschko addresses reporters in Kiev, Ukraine, on March 17.
Ukrainian troops stand guard in front of the Ukrainian Parliament building in Kiev on March 17.
A Ukrainian man applies for the National Guard at a mobile recruitment center in Kiev on March 17.
Civilians walk past riot police in Simferopol on March 17.
A Ukrainian soldier stands on top of an armored vehicle at a military camp near the village of Michurino, Ukraine, on March 17.
Policemen stand guard outside the regional state administration building in Donetsk, Ukraine, during a rally by pro-Russia activists March 17.
Armed soldiers stand guard outside a Ukrainian military base in Perevalne on March 17.
A man holds a Crimean flag as he stands in front of the Crimean parliament building in Simferopol on March 17.
Crimeans holding Russian flags celebrate in front of the parliament building in Simferopol on Sunday, March 16.
A Ukrainian police officer tries to shield himself from a road block thrown by pro-Russia supporters in Kharkiv, Ukraine, on March 16.
Pro-Russia demonstrators storm the prosecutor general's office during a rally in Donetsk on March 16.
A child casts her mother's ballot March 16 while holding a Russian flag at a polling station in Simferopol.
A pro-Russian soldier, with the Russian flag behind him, mans a machine gun outside an Ukrainian military base in Perevalne on Saturday, March 15.
A pro-Russian soldier guards the perimeter outside an Ukrainian military base in Perevalne on March 15.
Russian flags wave in front of a monument dedicated to Soviet Union founder Vladimir Lenin during a pro-Russia rally in Simferopol's Lenin Square on March 15.
Evgenyi Batyukhov cries March 14 at the site where pro-Russian and pro-Ukrainian activists clashed the night before in Donetsk, Ukraine.
A line of policemen stand in front of a bus of pro-Ukrainian activists as pro-Russian supporters confront them during a rally in Donetsk on Thursday, March 13.
Pro-Russian supporters clash with pro-Ukrainian activists in Donetsk on March 13.
Recent Academy Award winner Jared Leto walks through Independence Square in Kiev on March 13. During his Oscars acceptance speech in early March, the actor spoke to protesters in Ukraine and Venezuela saying, "We're thinking of you tonight."
A member of a pro-Russian self-defense unit checks a polling station near Simferopol on March 13.
A Ukrainian soldier looks out of the window of a regional military building with a poster reading "Ukraine's armed forces wait for you!" in Donetsk on March 13.
An armed Cossack stands guard at a checkpoint on the road from Simferopol to Sevastopol on March 13.
A pro-Russian soldier speaks to a truck driver outside the Ukrainian infantry base in Perevalne on Wednesday, March 12.
Cossacks stand guard at Crimea's regional parliament building in Simferopol on March 12.
Soldiers are seen aboard the Ukrainian ship Slavutych in the harbor of Sevastopol on Tuesday, March 11.
Ukrainian naval officers board a boat in front of the Russian minesweeper Turbinist in Sevastopol's harbor on March 11.
People shout slogans during a pro-Russia rally in Donetsk on Sunday, March 9.
Ukrainian police detain a demonstrator during a pro-Russian rally in Donetsk on March 9.
Pro-Russia protesters remove a Ukrainian flag from a flagpole taken from a government building in Donetsk on March 9.
Cossacks and other pro-Russian forces stand guard outside a government building in Simferopol on Saturday, March 8.
Ukrainian soldiers load armored personnel carriers into boxcars in the western Ukrainian city of Lviv on March 8.
Armed men believed to be Russian military march in a village outside Simferopol on Friday, March 7.
Pro-Russia protesters demonstrate outside the Belbek air base, near Sevastopol, on Thursday, March 6.
A Ukrainian navy officer looks at the scuttled, decommissioned Russian vessel Ochakov from the Black Sea shore outside the town of Myrnyi, Ukraine, on March 6. Russian naval personnel scuttled the ship, blockading access for five Ukrainian naval vessels.
A member of the Russian military patrols around Perevalne on March 6.
Servicemen guard a checkpoint at a Ukrainian navy base in Perevalne on March 6.
Ukrainian troops guard the Belbek air base on March 6.
A woman walks past barricades March 6 that were set up by anti-government protesters in Kiev's Independence Square.
A sailor guards the Ukrainian Navy ship Slavutych in the Bay of Sevastopol on Wednesday, March 5.
People wait in line for food distribution in Independence Square on March 5.
Ukrainian sailors carry meat to their vessel in the Sevastopol harbor on March 5.
Riot police stand at the entrance of a regional administrative building during a rally in Donetsk on March 5.
A Ukrainian police officer gives instructions to members of the media in front of the business class lounge of the Simferopol airport on March 5.
Pro-Russia demonstrators wave a Russian flag after storming a regional administrative building in Donetsk on March 5.
Demonstrators break a police barrier as they storm a regional administrative building in Donetsk on March 5.
Ukrainian military recruits line up to receive instructions in Kiev's Independence Square on Tuesday, March 4.
People stand on the Ukrainian Navy ship Slavutych while it's at harbor in Sevastopol on March 4. Mattresses were placed over the side of the ship to hinder any attempted assault.
Ukrainian troops watch as a Russian navy ship blocks the entrance of the Ukrainian navy base in Sevastopol on March 4.
A woman photographs pro-Russian soldiers guarding Ukraine's infantry base in Perevalne on March 4.
U.S. Secretary of State John Kerry, wearing a blue scarf, visits a shrine March 4 for the people who were killed in February during anti-government protests in Kiev.
Yuli Mamchun, the commander of the Ukrainian military garrison at the Belbek air base near Sevastopol, salutes on March 4.
Russian soldiers stand guard at the Belbek air base on March 4.
Ukrainian military members march at the Belbek air base on March 4.
Russian soldiers fire warning shots to keep back Ukrainian military members at the Belbek air base on March 4.
A Ukrainian airman puts the Ukrainian national flag over the gate of the Belbek air base as they guard what's left under their control on March 4.
Russian soldiers aim a grenade launcher and machine gun as they guard positions at the Belbek air base on March 4.
Ukrainian seamen stand guard on the Ukrainian navy ship Slavutych in the Sevastopol harbor on Monday, March 3.
Oleg, a Ukrainian soldier, kisses his girlfriend, Svetlana, through the gates of the Belbek base entrance on March 3. Tensions are high at the base, where Ukrainian soldiers were standing guard inside the building while alleged Russian gunmen were standing guard outside the gates.
Wives of Ukrainian soldiers walk past Russian soldiers to visit their husbands guarding a military base in Perevalne on March 3.
A Russian soldier guards an area outside Ukraine's military base in the village of Perevalne on March 3.
A sailor looks out a window near the entrance to the Ukrainian navy headquarters in Sevastopol on March 3.
Armed men in military uniform walk outside a Ukrainian military unit near Simferopol on Sunday, March 2. Hundreds of armed men in trucks and armored vehicles surrounded the Ukrainian base Sunday in Crimea, blocking its soldiers from leaving.
Soldiers walk outside a Ukrainian military base in Perevalne as a local resident waves a Russian flag March 2.
Demonstrators shout during a rally in Kiev's Independence Square on March 2.
Ukrainian soldiers, left, and unidentified gunmen, right, stand at the gate of an infantry base in Perevalne on March 2.
Ukrainian soldiers guard a gate of an infantry base in Perevalne on March 2.
A woman cries during a rally in Independence Square on March 2.
Protesters hold flags of the United States, Germany and Italy during a rally in Independence Square on March 2.
People attend a morning prayer service at Independence Square on March 2.
A soldier and a truck driver unload bread outside the Ukranian navy headquarters in Sevastopol on March 2.
Heavily armed troops, displaying no identifying insignia and who were mingling with local pro-Russian militants, stand guard outside a local government building in Simferopol on March 2.
A woman waits in front of unidentified men in military fatigues who were blocking a base of the Ukrainian frontier guard unit in Balaklava, Ukraine, on Saturday, March 1.
U.S. President Barack Obama, in the Oval Office of the White House, talks on the phone March 1 with Russian President Vladimir Putin.
Troops stand guard in Balaklava on March 1.
Heavily armed soldiers displaying no identifying insignia maintain watch in Simferopol on March 1.
People gather around the coffin of a man who was killed during clashes with riot police in Independence Square.
Pro-Russian activists hold Russian flags during a rally in the center of Donetsk on March 1.
Pro-Russian activists clash with Maidan supporters as they storm the regional government building in Kharkiv, Ukraine, on March 1.
A protester stands at a memorial March 1 for the people killed in clashes at Independence Square.
Armed men patrol outside the Simferopol International Airport on Friday, February 28.
An image provided to CNN by a local resident shows Russian tanks on the move in Sevastopol.
Russian troops block a road February 28 toward the military airport in Sevastopol. The Russian Black Sea Fleet is based at the port city.
Armed men stand guard in front of a building near the Simferopol airport on February 28.
An armed man wearing no identifying insignia patrols outside Simferopol International Airport on February 28.
Police stand guard outside the Crimea regional parliament building Thursday, February 27, in Simferopol. Armed men seized the regional government administration building and parliament in Crimea.
Police intervene as Russian supporters gather in front of the parliament building in Simferopol on February 27.
A man adds fuel to a fire at a barricade in Independence Square on February 27. Dozens of people were killed during clashes between security forces and protesters.
Pro-Russia demonstrators wave Russian and Crimean flags in front of a local government building in Simferopol on February 27.
Barricades in front of a government building in Simferopol on February 27 hold a banner that reads: "Crimea Russia." There's a broad divide between those who support the pro-Western developments in Kiev and those who back Russia's continued influence in Crimea and across Ukraine.
Protesters stand in front of a government building in Simferopol on February 27. Tensions have simmered in the Crimea region since the ouster of Ukrainian President Viktor Yanukovych.
Protesters in support of the president's ouster rally in Independence Square, which has been the center of opposition, on Wednesday, February 26.
Security forces stand guard during clashes between opposing sides in front of Crimea's parliament building in Simferopol on February 26.
Pro-Russian demonstrators, right, clash with anti-Russian protesters in front of a government building in Simferopol on February 26.
A police officer gets pulled into a crowd of Crimean Tatars in Simferopol on February 26. The Tatars, an ethnic minority group deported during the Stalin era, rallied in support of Ukraine's interim government.
A man places flowers at a barricade near Independence Square on February 26.
On February 26 in Kiev, a woman holds a photograph of a protester killed during the height of tensions.
Police guard a government building in Donetsk on February 26.
Protesters remove a fence that surrounds Ukraine's parliament in Kiev on February 26.
People sing the Ukrainian national anthem at Independence Square on Monday, February 24.
Gas masks used by protesters sit next to a barricade in Independence Square on February 24.
A woman cries February 24 near a memorial for the people killed in Kiev.
People wave a large Ukrainian flag in Independence Square on Sunday, February 23.
Two pro-government supporters are made to pray February 23 in front of a shrine to dead anti-government protesters.
A man and his daughter lay flowers at a memorial for protesters killed in Independence Square.
Former Prime Minister Yulia Tymoshenko speaks at Independence Square on Saturday, February 22, hours after being released from prison. Tymoshenko, considered a hero of a 2004 revolution against Yanukovych, was released after 2½ years behind bars.
Tymoshenko is greeted by supporters shortly after being freed from prison in Kharkiv on February 22.
A protester guards the entrance to Yanukovych's abandoned residence outside Kiev on February 22.
Anti-government protesters guard the streets next to the presidential offices in Kiev on February 22.
Anti-government protesters drive a military vehicle in Independence Square on February 22. Many protesters said they wouldn't leave the square until Yanukovych resigned.
Ukrainian lawmakers argue during a session of Parliament on Friday, February 21.
Men in Kiev carry a casket containing the body of a protester killed in clashes with police.
Protesters cheer after news of an agreement between the opposing sides in Kiev on February 21.
