(CNN) -- A top U.N. official arrived in Damascus on Tuesday in an attempt to "draw attention to the deteriorating humanitarian situation in Syria," the United Nations said.
Valerie Amos, the U.N.'s under-secretary-general for humanitarian affairs and emergency relief coordinator, is on a three-day trip to Syria and Lebanon.
"The humanitarian situation in Syria has further worsened in recent weeks as fighting has spread in Damascus and to Aleppo and other cities," the United Nations said. "Two million people are now estimated to have been affected by the crisis, and over one million have been internally displaced."
She met with Prime Minister Wael al-Halaki, Syrian TV reported.
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In Lebanon, Amos will meet with families who fled Syria and will talk with the government and humanitarian agencies about how to best support the refugees, her office said.
Members of the Free Syrian Army react as they fire a homemade rocket toward regime forces in Deir al-Zor on Sunday, June 16. Tensions in Syria flared in March 2011 during the onset of the Arab Spring, escalating into an ongoing civil war. View the most compelling images taken since the start of the conflict.
Syrian civil war in photos
Across Syria, the reports of relentless bloodshed mount by the hour.
At least 70 people were killed Tuesday, the Local Coordination Committees of Syria opposition group said. The toll included 10 children and three women.
Syria said the assistant director of the Health Directorate in Daraa was "assassinated" by "an armed terrorist group" Tuesday. Attackers waited for Dr. Ma'moun al-Zoubi at the garage of his building "and opened fire on him, causing his martyrdom," the state-run Syrian Arab News Agency (SANA) reported.
In recent days, fierce shelling in and around dissident strongholds has aggravated the humanitarian crisis, opposition activists said.
In the Homs suburb of Rastan, residents pleaded for intervention "to break the siege of the disaster-stricken city, which has no access to any of life's essentials including bread, water, baby formula or electricity," the LCC said.
Syria said authorities "arrested dozens of terrorists" in Aleppo and destroyed seven cars equipped with machine guns. Armed forces also killed and injured "terrorists" in an area called al-Kura al-Ardiya, SANA reported.
Foreign ministers of Islamic countries have agreed to suspend Syria from the Organization of Islamic Cooperation, ahead of the group's two-day emergency summit this week.
The Syrian civil war is a key topic on the agenda of the summit, which opens Tuesday in the Islamic holy city of Mecca, Saudi Arabia.
Read more: Syria expected to dominate Islamic summit
"There is an agreement to support the Syrian people, and standing by their side, we are agreeing on freezing the membership of Syria until it retains its balance," Tunisian Foreign Minister Rafiq Abdul Salam said, according to the official Saudi Press Agency.
The heads of state must approve the suspension before it can take effect.
Meanwhile, a Syrian presidential adviser was scheduled to visit China on Tuesday, the Chinese Foreign Ministry said. China, along with Russia, has repeatedly vetoed attempts at the U.N. Security Council to take stronger action against Syrian President Bashar al-Assad's regime.
A spokesman for the ministry said Bouthaina Shaaban, al-Assad's political and media adviser, will meet with Chinese Foreign Minister Yang Jiechi.
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"To promote the political solution to the Syria problem, China has always actively balanced its work between the Syrian government and the opposition to urge Syria to put to practice (former Arab League and U.N. envoy Kofi) Annan's suggestions and advice by the U.N. Security Council -- immediately cease fire and all violent actions, protect civilians and resolve the crisis through talks," the spokesman said.
"China is also considering inviting members of the Syrian opposition to visit," the ministry said in a statement.
The United States, meanwhile, lifted sanctions against the former Syrian Prime Minister Riyad Hijab, who recently broke with the regime.
"Recent civilian and military defections from the Assad regime are further indications that the government is crumbling and losing its grip on power," David S. Cohen, under secretary for terrorism and financial intelligence at the U.S. Treasury, said in a statement. "The United States encourages other officials within the Syrian government, in both the political and military ranks, to take similarly courageous steps to reject the Assad regime and stand with the Syrian people."
While the war rages on, Syrian rebels claimed one of their biggest achievements yet Monday -- the shooting down of a military jet.
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Video posted by the rebel forces shows a jet framed in a cloudless sky being shot at, catching fire and falling out of frame.
"A MiG warplane shot down in Mouhassen!" says an excited man off-camera, citing a location in Deir Ezzor. "God is great!"
The government denied the jet was shot down, blaming the crash instead on a "technical failure." It said that the jet fighter was on an "ordinary training flight" and that the pilot ejected safely.
The search for the pilot was still under way late Monday, according to SANA.
A few hours after the rebels made their claim, the opposition LCC posted a video showing what it said was the pilot, whom it identified as Col. Mufeed Mohamad Suleiman. No identification card was shown.
The man, dressed in civilian clothes and surrounded by armed men, called on other military officers to defect. He said the rebels had provided first aid for the bruises on his face that he suffered in the parachute jump.
A man who identified himself as a rebel captain said the pilot would be treated as a prisoner of war.
CNN cannot independently confirm the authenticity of either video.
A report Monday from opposition activists underscored the difficulty in resolving the Syrian crisis. It said that Syrian forces in Damascus had publicly executed at least 10 people in one town, and that residents were unable to reach the bodies because "regime forces have been firing at anything that moves," the LCC said.
The Syrian crisis has claimed roughly 17,000 lives since it erupted last year, U.N. Secretary-General Ban Ki-moon said last month. Opposition activists have put the toll at more than 20,000.
Regional group votes to suspend Syria; rebels claim downing of jet
CNN's Holly Yan, Brian Walker and Salma Abdelaziz contributed to this report.
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Syrian rebels leave their position in the northwestern town of Maaret al-Numan on Thursday, June 13. The White House said that the Syrian government has crossed a "red line" with its use of chemical weapons and announced it would start arming the rebels.
Forces loyal to Syrian President Bashar al-Assad are seen near Qusayr on Thursday, May 30.
Syrian rebels take position in a house during clashes with regime forces in the old city of Aleppo on May 22.
Syrian army soldiers take control of the village of Western Dumayna north of the rebel-held city of Qusayr on Monday, May 13. Syrian troops captured three villages in Homs province, allowing them to cut supply lines to rebels inside Qusayr town, a military officer told AFP.
Rebel fighters fire at government forces in the northern Syrian city of Aleppo on Sunday, May 12.
Searchers use a flashlight as they look for survivors among the rubble created by what activists say was a missile attack from the Syrian regime, in Raqqa province, Syria, on April 25.
A Kurdish fighter from the "Popular Protection Units" (YPG) takes position inside a building in the majority-Kurdish Sheikh Maqsood area of Aleppo, on Apri. 21.
People walk past destroyed houses in the northern Syrian town of Azaz on Sunday, April 21.
Free Syrian Army fighters take positions prior to an offensive against government forces in the Khan al-Assal area, near Aleppo on Saturday, April 20.
Men inspect damage at a house destroyed in an airstrike in Aleppo on April 15.
Syrian and Kurdish rebel fighters walk in the Sheikh Maqsud district of Aleppo on April 14.
A female rebel monitors the movement of Syrian government forces in Aleppo's Sheikh Maqsud neighborhood on April 11.
A rebel runs to avoid sniper fire from Syrian government forces in Aleppo on Thursday, April 11.
Syrian rebels observe the movement of Syrian government forces around Al-Kendi hospital in Aleppo on Wednesday, April 10.
Rescue teams and security forces check out the scene of a deadly car bomb explosion in Damascus on April 8.
The fighting has taken a toll on buildings in Aleppo's Saladin district, seen here on April 8.
A Syrian rebel runs for cover in Deir ez-Zor on April 2.
A rebel checks for snipers across the street toward the Citadel in Aleppo, Syria, on Saturday, March 30, in this photo taken by iReporter Lee Harper.
A Free Syrian fighter mourns the death of a friend in Aleppo on March 30, in this photo taken by iReporter Lee Harper.
A Syrian opposition fighter runs for cover from Syrian army snipers in Aleppo on Wednesday, March 27.
A Syrian girl covers her face to protect herself from fumes as a street covered with uncollected garbage is fumigated in Aleppo on Sunday, March 24.
A Syrian man and his family drive past damaged buildings in Maarat al-Numan, on Wednesday, March 20.
Syrians carry the body of a Syrian army soldier during a funeral ceremony in Idlib province on Tuesday, March 19.
Syrian rebels take position in Aleppo, the largest city in the country, on March 11.
Syrian men search for their relatives amongst the bodies of civilians executed and dumped in the Quweiq River on March 11.
A Free Syrian Army fighter looks back as smoke rises during fighting between rebel fighters and forces loyal to Syria's President Bashar al-Assad on the outskirts of Aleppo on Saturday, March 2.
Residents read Shaam News newspapers published by the Free Syrian Army in Aleppo on March 2.
A member of the Free Syrian Army reacts to the death of a comrade who was killed in fighting, at Bustan al Qasr cemetery in Aleppo on Friday, March 1.
A rebel fighter throws a home-made grenade at Syrian government forces in Aleppo on February 16.
A member of the Free Syrian Army stands with his weapon as he looks at a rainbow in Aleppo on February 16.
A Syrian woman looks through a bus window in Aleppo on February 14.
Free Syrian Army fighters walk through a dust-filled stairwell in Damascus on February 7.
A Syrian rebel gestures at comrades from inside a broken armored personnel carrier in Al-Yaqubia on February 6.
A rebel fighter throws a hand grenade inside a Syrian Army base in Damascus on February 3.
People stand in the dust of a building destroyed in an airstrike in Aleppo, Syria on February 3.
Free Syrian Army fighters run as they enter a Syrian Army base during heavy fighting in the Arabeen neighborhood of Damascus on February 3.
An unexploded mortar shell fired by the Syrian Army sits lodged in the ground in Damascus on January 25.
Fighters from Fateh al Sham unit of the Free Syrian Army fire on Syrian Army soldiers at a check point in Damascus on January 20.
A Free Syrian Army fighter walks between buildings damaged during Syrian Air Force strikes in Damascus on January 19.
A Syrian rebel fighter tries to locate a government jet fighter in Aleppo on January 18.
Syrian rebels launch a missile near the Abu Baker brigade in Albab on January 16.
A Syrian boy walks near rubbish next to tents at a refugee camp near the northern city of Azaz on the Syria-Turkey border, on January 8.
Syrians look for survivors amid the rubble of a building targeted by a missile in Aleppo on January 7.
A father reacts after hearing of a shelling by forces loyal to Syria's President Bashar al-Assad in Aleppo on January 3.
A patient smokes a cigarette at Dar Al-Ajaza psychiatric hospital in Aleppo on December 18, 2012. The psychiatric ward, housing around 60 patients, has lacked the means to function properly since fighting broke out there in July.
Syrians mourn a fallen rebel fighter at a rebel base in the al-Fardos area of Aleppo on December 8.
Members of Liwa (Brigade) Salahadin, a Kurdish military unit fighting alongside rebel fighters, monitor the area in the besieged district of Karmel al-Jabl in Aleppo on December 6.
A member of Liwa Salahadin aims at a regime fighter in the besieged district of Karmel al-Jabl in Aleppo on December 6.
Two young boys sit underneath a washline in a refugee camp on the border between Syria and Turkey near Azaz on December 5.
The bodies of three children, who were allegedly killed in a mortar shell attack that landed close to a bakery in Aleppo, on December 2, are laid out for identification by family members at a makeshift hospital at an undisclosed location of the city.
Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo on December 1 as fighting continues through the night.
Damaged houses in Aleppo are seen after an airstrike on November 29.
A Syrian rebel mourns the death of a comrade in Maraat al-Numan on November 20.
Syrians protesters stand on Assad's portrait during an anti-regime demonstration in Aleppo on November 16.
A Syrian rebel takes cover during fighting against Syrian government forces in Aleppo on November 15.
Syrian opposition fighter Bazel Araj, 19, sleeps next to his pistol in Aleppo on November 11.
A rebel fighter fires at a Syrian government position in Aleppo on November 6.
A Syrian rebel leaps over debris left in the street while running across a "sniper alley" near the Salahudeen district in Aleppo on November 4.
Rebels hold their position in the midst of a battle on November 3 in Aleppo.
A man cries while being treated in a local hospital in a rebel-controlled area of Aleppo on October 31.
A man is treated for wounds after a government jet attacked the Karm al-Aser neighborhood in eastern Aleppo on October 31.
A Syrian rebel interrogates a handcuffed and blindfolded man suspected of being a pro-regime militiaman in Aleppo on October 26.
Smoke rises from a fuel station following a mortar attack as Syrian women walk on a rainy day in the Arqub neighborhood of Aleppo on October 25.
A Syrian rebel fires at an army position in the Karm al-Jabal district of Aleppo on October 22.
A wounded Syrian boy sits on the back of a truck carrying victims and wounded people to a hospital following an attack by regime forces in Aleppo on October 21.
A man lies on the ground after being shot by a sniper for a second time as he waits to be rescued by members of the Al-Baraa Bin Malek Battalion, part of the Free Syria Army's Al-Fatah brigade, in Aleppo on October 20.
Syrian army soldiers run for cover during clashes with rebel fighters at Karam al-Jabal neighborhood of Aleppo on October 20.
Smoke rises after a Syrian Air Force fighter jet fired missiles at the suburbs of the northern province of Idlib on October 16.
A Syrian opposition fighter stands near a post in Aleppo on October 11.
A Syrian man mourns the death of his father, who was killed during a government attack in Aleppo on October 10.
A rebel fighter is carried by his friends and laid on a gurney to be treated for gunshot wounds sustained during heavy battles with government forces in Aleppo on October 1.
Syrian rebels help a wounded comrade to an Aleppo hospital after he was injured in a Syrian army strike on September 18.
Free Syria Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16.
A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in Aleppo on September 14.
A woman walks past a destroyed building in Aleppo on September 13.
Free Syrian Army fighters battle during street fighting against Syrian army soldiers in Aleppo on September 8.
A Syrian man wounded by shelling sits on a chair outside a closed shop in Aleppo on September 4.
A woman sits in her wheelchair next to her house, damaged by a Syrian air raid, near Homs on August 26.
Members of the Free Syrian Army clash with Syrian army soliders in Aleppo's Saif al-Dawla district on August 22.
A man mourns in front of a field hospital on August 21 in Aleppo.
Wounded civilians wait in a field hospital after an air strike on August 21 in Aleppo.
People pray during the funeral of a Free Syrian Army fighter, Amar Ali Amero, on August 21.
A man cries near the graves of his two children killed during a recent Syrian airstrike in Azaz on August 20.
A Syrian woman holds her dead baby as she screams upon seeing her husband's body being covered following an airstrike by regime forces on the town of Azaz on August 15.
A Syrian rebel runs in a street of Selehattin during an attack on the municipal building on July 23.
Syrian rebels hunt for snipers after attacking the municipality building in the city center of Selehattin on July 23.
Members of the Free Syrian Army's Mugaweer (commandos) Brigade pay their respects in a cemetery on May 12 in Qusayr.
Syrian rebels take position near Qusayr on May 10.
A Free Syrian Army member takes cover in underground caves in Sarmin on April 9.
Rebels prepare to engage government tanks that advanced into Saraquib on April 9.
Men say prayers during a ceremony in Binnish on April 9.
A young boy plays with a toy gun in Binnish on April 9.
A Free Syrian Army rebel mounts his horse in the Al-Shatouria village near the Turkish border in northwestern Syria on March 16, a year after the uprising began.
Syrian refugees walk across a field before crossing into Turkey on March 14.
A rebel takes position in Al-Qsair on January 27.
A protester in Homs throws a tear gas bomb back towards security forces, on December 27, 2011.
A man stands under a giant Syrian flag outside the Umayyad Mosque in Damascus on December 24, 2011.
A member of the Free Syrian Army looks out over a valley in the village of Ain al-Baida on December 15, 2011.
Members of the Free Syrian Army stand in an valley near the village of Ain al-Baida, close to the Turkish border, on December 15, 2011.
Displaced Syrian refugees walk through an orchard adjacent to Syria's northern border with Turkey on June 14, 2011, near Khirbet al-Jouz.
A Syrian man holds up a portrait of President Bashar al-Assad during a rally to show support for the president in Damascus on April 30, 2011.
Syrians rally to show their support for President Bashar al-Assad in Damascus on April 30, 2011.
A screen grab from YouTube shows thick smoke rising above as Syrian anti-government protesters demonstrate in Moaret Al-Noman on April 29, 2011.
A screen grab from YouTube shows Syrian anti-government protesters run for cover from tear gas fired by security forces in Damascus on April 29, 2011, during the "Day of Rage" demonstrations called by activists to put pressure on al-Assad.
Syrians wave their national flag and hold portraits of al-Assad during a rally to show their support for their leader in Damascus on March 29, 2011.
A woman sits by the hospital bed of a man allegedly injured when an armed group seized rooftops in Latakia on March 27, 2011, and opened fire at passers-by, citizens and security forces personnel according to official sources.
Syrian protesters chant slogans in support of al-Assad during a rally in Damascus on March 25, 2011.
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