Syrian opposition activists accused the regime on Sunday of a gruesome "massacre" after several hundred bodies were found in a small town near Damascus following a ferocious army assault.

Grisly videos issued by opposition militants showed dozens of charred and bloodied bodies lined up in broad daylight in a graveyard, and others lying wall-to-wall in rooms in a mosque in the town of Daraya.

At least 320 people were killed in a five-day onslaught on Daraya by troops battling to crush insurgents who have regrouped in the outskirts of the capital, according to a toll from the Syrian Observatory for Human Rights.

The Local Coordination Committees, a network of activists on the ground, described it as a "massacre" by President Bashar al-Assad's regime and said many victims had been summarily executed and their bodies burnt.

"The shabiha (pro-regime) militias... have been transformed into a killing machine that threatens the Syrian people and our future," it said.

Human rights groups have accused the regime of committing many atrocities since the uprising against Assad's rule erupted in March last year, and a UN panel said earlier this month it was guilty of crimes against humanity.

In all, at least 183 people were killed nationwide on Saturday, the Observatory said, as the brutal conflict that has convulsed Syria for more than 17 months showed no signs of abating.

In the first reaction by a world power, Britain said that if confirmed, the Daraya massacre "would be an atrocity on a new scale, requiring unequivocal condemnation from the entire international community."

A video posted by militants on YouTube showed dozens of bodies in dimly lit rooms, with a commentary referring to "an odious massacre committed by the gangs of the Assad regime in the Abu Sleiman Addarani Mosque."

In another LCC video, Daraya's dead, among them at least two children, were shown being prepared for burial, their bodies lying in a hastily dug trench covered with blankets and strewn with palm fronds.

State media said Daraya, a mainly Sunni Muslim town of some 200,000 people, was being "purified of terrorist remnants."

A pro-government television said "terrorists" carried out the attacks, as it interviewed residents including traumatised children and showed a number of bloodied bodies lying in the streets.

"Our valiant armed forces cleared Daraya of the remnants of armed terrorist groups which committed crimes that traumatised the citizens of the town and destroyed public and private property," government newspaper Ath-Thawra said.

Activists described the Daraya offensive as a bid to crush "once and for all" the insurgency in Damascus after rebel Free Syrian Army fighters regrouped to the southern outskirts following an army offensive to retake the city last month.

The LCC accused the regime of blockading Daraya to choke off supplies and indiscriminately bombarding the town with heavy weapons and warplanes.

"Afterwards the gangs of killers entered the town and carried out summary executions, before dismembering and setting fire to the bodies."

Reports by activists cannot be independently confirmed because of severe restrictions on media operating in Syria.

Assad's Alawite-led regime insists it is fighting foreign "terrorists" aided by regional Sunni Muslim rivals and the West, but has been hit by a wave of defections, including several high-ranking officials, as the violence escalates.

On Sunday, Vice President Faruq al-Shara – the regime's top Sunni Muslim official – made his first public appearance in over a month, following rumours that he had tried to defect and was under house arrest.

Shara and Assad held talks with the head of the Iranian parliament's foreign policy committee, Aladin Borujerdi, who vowed that Tehran will "stick by our Syrian brothers".

"We see Syria's security as our security," he said, Iran's state-owned IRNA news agency said.

Tehran – Damascus's staunchest ally – has said it will submit a plan for ending the conflict to a Non-Aligned Movement summit it is hosting on Thursday and Friday.

Syrian Foreign Minister Walid Muallem also said any negotiations between the government and the opposition would only begin "after purging Syria of armed groups," IRNA reported.

Tehran's initiative comes as its foes in the West seek to ramp up the pressure on Damascus, with Washington and London threatening action if it uses its chemical weapons and Paris voicing support for a partial no-fly zone.

The Observatory also reported shelling or air strikes in other parts of the country including the battered northern city of Aleppo and Daraa in the far south, the cradle of the uprising.

A veteran Japanese war reporter shot dead covering the conflict in Aleppo, the fourth foreign journalist to be killed in Syria, was hit by nine bullets, according to results of an autopsy published Sunday.

A report by UN investigators this month said government forces and their militia allies had committed crimes against humanity and that rebels had also carried out war crimes, although on a lesser scale.

In particular, it held government forces responsible for a massacre in the central town of Houla in May when 108 civilians, including 49 children, were killed in an atrocity that shocked the world.

August is already the deadliest single month of the conflict with at least 4,000 people killed, according to the Observatory, while around 25,000 have died since March 2011.

The United Nations puts the death toll at more than 17,000 and has warned of a major humanitarian crisis with more than 200,000 refugees fleeing to neighbouring countries and 2.5 million in need inside Syria.

New international envoy Lakhdar Brahimi, who takes over from Kofi Annan next month, said on Friday he was "scared" of the enormity of the task he faces to try to end what he describes as civil war.

Syria warned Brahimi on Sunday not to follow the same path as Annan, with Ath-Thawra accusing the former UN chief who quit this month after the failure of his peace plan of "bowing to US and Western pressure."

Damascus said last week it would cooperate with Brahimi to try to pave the way for "national dialogue," while also suggesting it was ready to discuss Assad's exit as part of any negotiated solution.