Over 300 civilians have been massacred by Government troops in a Damascus suburb, according to opposition forces.
President Bashar al-Assad’s troops had launched a ferocious assault on the suburb of Darayya, with state TV saying that the area was being “cleansed of terrorists”.
However, rebel forces claim that civilians were summarily executed by troops during the attack.
Video footage of the dead has emerged, with the bodies lined up side by side.
They were found in basements and cellars, the BBC reported.
"Assad's army has committed a massacre in Daraya," Abu Kinan, an activist in the town told Reuters.
"In the last hour, 122 bodies were discovered and it appears that two dozen died from sniper fire and the rest were summarily executed by gunshots from close range.”
The activist added in a telephone interview with the news agency that he’d seen an eight-year-old girl shot by snipers while she was in a car with her parents.
The Foreign Office warned that violence in Syria had reached a "new scale" amid claims that more than 300 people had been massacred.
Foreign Office minister Alistair Burt said: "I am deeply concerned by emerging reports of a brutal massacre of civilians in Darayya, in the outskirts of Damascus.
"The Syrian regime's appalling repression of its people, over more than 17 months, has left little space for independent observers to operate in Syria.
"This makes it extremely difficult to verify what took place yesterday. Opposition groups report that over 300 people, including women and children, were killed and that some were shot at close range.
"If confirmed it would be an atrocity on a new scale, requiring unequivocal condemnation from the entire international community.
"It would make yesterday the bloodiest day since the unrest in Syria began in March 2011, with over 400 killed across the country."
Mr Burt said the development highlighted the urgent need for international action to bring an end to the violence. Russia and China have blocked UN Security Council resolutions that would increase pressure on the regime.
Meanwhile, Syrian Vice-President Farouq al-Shara has greeted an Iranian delegation in Damascus, ending weeks of speculation that he had defected.
However, Sky’s Foreign Affairs Editor Tim Marshall, reporting from Damascus, explained that it’s impossible to verify who the dead people are or how they died.
He has seen the video footage of the bodies, but said: “We don’t know when these pictures were shot. They show men in civilian clothes, of fighting age.
"They’ve all been gathered up by opposition fighters and filmed. Clearly there’s been a large loss of life at some point, we cannot verify where it is. We don’t know how they can get all these people together in one place so quickly.
“There’s no hard evidence [of a massacre].”
He added that a pattern had emerged of opposition reports claiming a massacre shortly before major UN meetings about Syria in New York – and there is such a meeting this week.
Syrian boys play next to a house destroyed in a Syrian government bombing last week that killed more than 40 people, in Azaz, on the outskirts of Aleppo, Syria, Friday, Aug. 24, 2012. (AP Photo/Muhammed Muheisen)
Lt. Gen. Babacar Gaye, the head of the failed United Nations Supervision Mission in Syria, departs for Lebanon in a UN convoy from his residence at the Dama Rose hotel in Damascus, Syria, Saturday, August 25, 2012. The observers' deployment in April that had intended to supervise a truce that never took hold, was one of the only concrete achievements in former Special Envoy Kofi Annan's attempts to halt the country's escalating civil war. There sign in Arabic reads "armed forces officer's club." (AP Photo/Bassem Tellawi)
In this Friday, Aug. 24, 2012 photo, Mohammed Alhassan, 20, a former member of the Syrian security forces is currently jailed in a makeshift prison run by rebels in a former elementary school in Al-Bab on the outskirts of Aleppo, Syria. Many improvised detention centers have sprung up as rebels wrest cities from army control, but these facilities fall under no national or regional authority, causing concern among rights groups. (AP Photo/Muhammed Muheisen)
In this Wednesday, March 21, 2012 file photo, a Syrian woman, right, kisses a soldier from the Free Syrian Army, in front a destroyed Syrian army forces tank which was attacked during clashes between the Syrian government forces and the Syrian rebels in Rastan, Syria. As Syria's rebels gain ground across the country, the fighters are trying to win the hearts and minds of their countrymen and the international community --capturing army diesel trucks to give to impoverished villagers, working to restore damaged power lines and smuggling in foreign journalists. (AP Photo, File)
In this Thursday, June 14, 2012 file photo, members of the Free Syrian Army receive a first aid lesson in the Jabal Al- Zawiya area near Idlib, Syria. As Syria's rebels gain ground across the country, the fighters are trying to win the hearts and minds of their countrymen and the international community -- capturing army diesel trucks to give to impoverished villagers, working to restore damaged power lines and smuggling in foreign journalists. (AP Photo/Khalil Hamra, File)
Lebanese cameramen take cover behind an armored personnel carrier of the Lebanese army after they were targeted by a sniper during clashes that erupted between pro and anti-Syrian regime gunmen in the northern port city of Tripoli, Lebanon, Friday, Aug. 24, 2012. The latest round of fighting first erupted on Monday in northern Lebanon and at least 15 have been killed in Tripoli this week and more than 100 have been wounded in fighting that is a spillover from Syria's civil war. (AP Photo/Hussein Malla)
Relatives and mourners carry the coffin of Free Syrian Army fighter Emad Nimeh, 18, who was killed today in an airstrike, during his funeral procession in Al-Bab, on the outskirts of Aleppo, Syria, Friday, Aug. 24, 2012. (AP Photo/Muhammed Muheisen)