Syrian President Bashar al-Assad said the foreign "conspiracy" against his country would be defeated, as his forces were accused of a bloody rampage in a town near Damascus that left hundreds dead.
"The Syrian people will not allow this conspiracy to achieve its objectives" and will defeat it "at any price," Assad said during a meeting in Damascus with a top official from Iran, Syria's chief regional ally.
The Syrian leader has since March last year been trying through force to smother a popular uprising that has turned into a brutal civil war which has left thousands dead, seen more than 200,000 refugees fleeing to neighbouring countries and 2.5 million in need inside Syria.
The Syrian Observatory for Human Rights said at least 320 people were killed in a five-day onslaught on Daraya, a satellite town southwest of the capital, by troops battling to crush insurgents.
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 Daraya.
The Local Coordination Committees, a network of activists on the ground, described it as a "massacre" by 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 in its attempts to crush the uprising, and a UN panel said earlier this month it was guilty of crimes against humanity.
"Bodies were found in fields, basements and shelters and in the streets," Observatory director Rami Abdel Rahman told AFP.
He said 200 bodies had been identified so far, including 15 women and 14 children, and that many of the victims had died in shelling or were summarily executed.
In the first reaction by a world power, Britain said that if confirmed, the Daraya massacre "would be an atrocity on a new scale."
Activists issued graphic videos of the scenes in Daraya, one showing 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.
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Pro-government television Al-Dunia 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.
Meanwhile, the head of the Iranian parliament's foreign policy committee, Aladin Borujerdi, vowed that Tehran will "stick by our Syrian brothers."
"We see Syria's security as our security," he said in Damascus, where he met both Assad and Vice President Faruq al-Shara, Iran's state-owned IRNA news agency said.
Shara -- the regime's top Sunni Muslim official -- made his first public appearance in over a month when he was seen walking and talking with Borujerdi, following opposition claims he had tried to defect and was under house arrest.
Borujerdi also said that Assad had told him that Syrian Prime Minister Wael al-Halqi and Foreign Minister Walid Muallem would attend the August 30-31 Non-Aligned summit which Tehran is hosting.
Iran is a staunch ally of Assad's regime but is being excluded from most international efforts aimed at ending the conflict in Syria.
Assad, in his talks with the visiting Iranian official, said Syria is continuing "its strategy of resistance."
"What is happening now is not only directed at Syria but the whole region. Because Syria is the cornerstone, foreign powers are targeting it so their conspiracy succeeds across the entire region."
Assad has long characterised the brutal conflict as a fight against foreign "terrorists" aided by the West and Syria's Sunni Muslim foes in the region, including Saudi Arabia.
Tehran has said it will submit a plan for ending the conflict to the Non-Aligned Movement summit.
The Iranian initiative comes as its foes in the West 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 on Sunday including the battered northern city of Aleppo and Daraa in the far south, the cradle of the anti-Assad uprising.
The Britain-based Observatory reported a total of at least 91 people killed countrywide on Sunday -- 61 civilians, 13 rebels and 17 loyalist soldiers.
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.