Syrian government troops have been accused of a massacre of civilians in Daraya, a town on the outskirts of Damascus which the regime retook from rebels on Saturday.

The Local Co-ordination Committees (LCC), an opposition network, claimed more than 200 bodies had been found in the working-class Sunni community south-west of the capital, and activists circulated a video appearing to show dozens of bodies, mostly of young men, lined up in dimly lit rooms, described in the commentary as being in the town's Abu Suleiman al-Durani mosque.

The government portrayed the attack as a counter-terrorism operation. "Our heroic armed forces cleansed Daraya from remnants of armed terrorist groups," the state news agency said.

Britain's foreign office minister for the Middle East, Alistair Burt, said that if the reports were verified, "it would be an atrocity on a new scale, requiring unequivocal condemnation from the entire international community".

The storming of Daraya on Saturday followed three days of heavy bombardment by government tanks and artillery, which the opposition said killed a further 70 people. The offensive appeared to be part of a larger struggle for control of the southern fringe of the capital. Local residents said that government tanks on the Damascus ring road shelled the neighbourhoods of al-Lawwan and Nahr Aisheh late into Saturday night and that there was also heavy fighting in the Ghouta suburbs to the east of the city.

The LCC said forces loyal to President Bashar al-Assad had killed a total of 440 people across Syria on Saturday alone. The Syrian Observatory for Human Rights, a British-based activist group drawing information from a network of monitors across Syria, put the nationwide death toll for the day at 370 including 174 civilians. If confirmed, it is one of the bloodiest days the country has suffered since the anti-Assad revolt broke out in March 2011.

It was impossible to verify such claims because of severe Syrian government restrictions on independent or foreign media coverage. A United Nations report this month into an earlier massacre at Houla found that the indiscriminate attacks against civilian populations and other atrocities were "state policy" and claimed Assad's forces and allied "Shabiha" militia were involved at the highest levels in "gross violation of international human rights".

The UN enquiry found that anti-Assad forces had also committed war crimes including "murder, extrajudicial execution and torture" but that these abuses "did not reach the gravity, frequency and scale of those committed by government forces and the Shabiha."

A new Amnesty Report on the fighting in Aleppo, Syria's largest city, also found: "The overwhelming majority of victims were killed in air strikes and artillery attacks by government forces." But it criticised rebels for using imprecise or indiscriminate weapons such as mortars and home-made rockets.

Activists in Daraya alleged that most of the victims had been summarily executed by government troops moving from house to house.

"Assad's army has committed a massacre in Daraya," said Abu Kinan, an activist in the town, told Reuters news agency by telephone, using an alias to protect himself from reprisals. "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," he said.

The activist said he witnessed the death of an eight-year-old girl, Asma Abu al-Laban, shot by army snipers while she was in a car with her parents.

"They were trying to flee the army raids. Three bullets hit her in the back and her parents brought her to a makeshift hospital. Nothing could be done for her," he said.

A thorough investigation of atrocity claims can only be carried out by the international criminal court in the the Hague if it is given a mandate by the UN security council, but that has been blocked by Russia, the Assad regime's principal backer and arms supplier, together with China. Moscow and Beijing have also vetoed resolutions threatening Assad with sanctions for non-compliance with a peace plan backed by the UN and the Arab League. The last UN monitors in Damascus left earlier this month when the security council failed to agree on a new mandate for them.

Western officials say they have largely given up on security council diplomacy and are stepping up their assistance to the fragmented opposition, though they say that assistance stops short of weapons. However, Saudi Arabia and Qatar are reported to be shipping arms to rebel groups as the conflict continues to escalate.

Iran is the regime's only major ally in the region and Iranian officials have said that Revolutionary Guards have fought alongside Assad's troops. Syria's Vice-President Farouk al-Sharaa met an Iranian delegation on Sunday, according to Syria's state news agency, marking his first appearance in several weeks. It put an end to opposition rumours that he had defected.

In the increasingly daunting search for a diplomatic solution, the UN and Arab League have appointed a new special envoy, a veteran Algerian diplomat, Lakhdar Brahimi, after the resignation of the former UN secretary general Kofi Annan. On Friday, Brahimi declared himself "honoured, flattered, humbled and scared" to be given the job.