(CNN) -- The newly elected head of Syria's opposition briefed European foreign ministers Monday on the crisis there, European Union foreign affairs representative Catherine Ashton said.
Ashton met Monday with Ahmed Moaz al-Khatib and pledged European support ahead of the monthly meeting of European Union foreign ministers.
"We want to help, but it's their country," she said.
Al-Khatib was to discuss his proposal for a political transition to end the hostilities that have left thousands of people dead during nearly 21 months of violence that Syrian President Bashar al-Assad has characterized as a fight against terrorism.
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Ashton said the world must take a stand against al-Assad's actions.
"It is important that we recognize the terrible things that have been happening in Syria and the responsibility that Assad has," Ashton said. "We said from the very beginning that it is no place to be in a position of power in your country if you respond to peaceful demonstrations with the murder of your citizens."
After hearing from al-Khatib, the foreign ministers released a statement saying, "The EU is seriously concerned about the potential use and transfer of chemical weapons in Syria and reminds the Syrian regime and other parties that, in case of their use, those responsible will be held accountable."
Syria has accused the United States of working to falsely accuse the country of using chemical weapons, Syrian state-run media said Monday.
"The U.S. administration has consistently worked over the past year to launch a campaign of allegations on the possibility that Syria could use chemical weapons during the current crisis," the Foreign Ministry wrote in letters to U.N. Secretary-General Ban Ki-moon, the state-run Syrian Arab News Agency reported.
"What raises concerns about this news circulated by the media is our serious fear that some of the countries backing terrorism and terrorists might provide the armed terrorist groups with chemical weapons and claim that it was the Syrian government that used the weapons," SANA quoted the letters as saying.
U.S. officials have said they have seen intelligence suggesting that Syrian military units may be preparing chemical weapons for use.
President Barack Obama has said any use of chemical weapons would cross a "red line," eliciting swift U.S. reaction.
The United States and European allies are using defense contractors to train Syrian rebels to secure chemical weapons stockpiles, a senior U.S. official and several senior diplomats said.
The training, in Jordan and Turkey, covers the monitoring and securing of stockpiles of weapons and materials, said the officials, who spoke on condition of anonymity because they were not authorized to publicly discuss the issue.
Some contractors are working with rebels in Syria to monitor sites, one of the officials said.
Opposition groups said fighting continued Monday across the country.
At least 142 people died Monday, 59 of them in Aleppo and 50 in Damascus and its suburbs, the Local Coordination Committees of Syria said.
In one Aleppo neighborhood, opposition activists said, they discovered 10 handcuffed and blindfolded corpses killed by government forces in recent weeks, the Syrian Observatory for Human Rights reported.
CNN is unable to confirm casualty reports as the government has severely restricted access by international journalists.
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. Tensions in Syria first flared in March 2011 during the onset of the Arab Spring, eventually escalating into a civil war that still rages. This gallery contains the most compelling images taken since the start of the conflict.
Syrian civil war in photos
The Observatory said a rebel group seized control of a government military base in northwest Syria.
The seizure occurred in Aleppo province, where rebel fighters from the jihadi al-Nusra Front, Muhikiri al-Sham and the al-Battar battalions took over three brigades and the command center of the 111th regiment in the Sheikh Sleiman area of western Reef Aleppo, the observatory said.
Two rebels and one soldier died; five other soldiers were captured, it said, adding that 140 soldiers and their officers fled.
The rebel forces represent a variety of interests. In Washington, State Department spokeswoman Victoria Nuland said Monday that U.S. officials were concerned "that al-Nusra is little more than a front for al Qaeda in Iraq who has moved some of its operations into Syria."
The State Department is planning to designate al-Nusra Front, a radical Islamist group, as a foreign terrorist organization, two U.S. officials told CNN last week.
The announcement is likely to come this week, the officials said.
The hope is to finalize the designation before the Friends of Syria meeting, which is slated to be held Wednesday in Morocco.
The goal of the designation is to isolate extremists groups in Syria while giving a boost to the new political opposition group unveiled last month in Doha, Qatar, they said.
Al-Nusra and several other groups announced their opposition to a new anti-government coalition last month. U.S. officials estimate al-Nusra members represent some 9% of rebel forces in Syria.
Officials said the group has not affiliated itself publicly with al Qaeda in an apparent effort to appear more mainstream.
In recent months, the al-Nusra Front has emerged as one of the most effective groups in the Syrian resistance, drawing on foreign fighters with combat experience in Iraq and elsewhere.
It has claimed responsibility for a number of complex attacks in Damascus and Aleppo, frequently involving suicide bombers.
CNN's Elise Labott and Samira Said contributed to this report.
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Syrian troops move into Dumayna on May 13.
Rebel fighters fire at government forces in the northern Syrian city of Aleppo on Sunday, May 12.
Smoke rises from an explosion in a Syrian village near the Israeli border on Tuesday, May 7.
Multiple explosions hit a Syrian village near the Israeli border on May 6.
A photo released by the Syrian Arab News Agency shows destruction from what is said was bomb attack in the Al-Hama area of Damascus on Sunday, May 5. According to the Syrian government, Israel launched an attack on a research center in the Damascus suburbs early Sunday.
People run for cover after what activists said was shelling by forces loyal to Syria's President Bashar al-Assad in Syria's Raqqa province, on May 3.
People walk past a damaged building and multiple destroyed cars at the site of an explosion in Damascus where at least 13 were killed on April 30.
Cleaning takes place following another explosion in an upscale Damascus neightborhood on Monday, April 29. Syrian Prime Minister Wael al-Halqi survived the bombing targeting his motorcade.
A smoke cloud rises from shelling on the the al-Turkman mountains in Syria's Latakia province on Thursday, April 25.
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 handout photograph from Syria's national news agency SANA shows damage and debris from a mortar attack in the suburb of Jarmana near Damascus, Syria, on Wednesday, April 24. The attack killed seven and wounded more than 25, according to activists and state media. No group claimed responsibility for the mortar fire, which SANA said hit a municipality office and a school building.
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 Syrian boy holds an AK-47 assault rifle in the streets of Aleppo on Sunday, 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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