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.