Syrian rebels claimed they shot down an army helicopter during fierce fighting in Damascus on Monday, saying it was to avenge the ‘massacre’ of over 330 people in the town of Daraya blamed on regime forces.

State television said the aircraft crashed near a mosque in the eastern district of Qaboon, where activists reported shelling, heavy fire by combat helicopters and clashes between government troops and Free Syrian Army rebels.

"It was in revenge for the Daraya massacre," Omar al-Qabooni, spokesman for the FSA's Badr battalion in Damascus told AFP, adding that rebels had found the body of the pilot after the helicopter crashed to the ground in a ball of flames.

The Syrian Observatory for Human Rights reported intensified shelling by regime forces after the helicopter came down in many areas in and around eastern Damascus where anti-regime sentiment is strong.

The FSA had previously said it shot down a Syrian warplane two weeks ago in the eastern province of Deir Ezzor and captured its pilot, but the claims cannot be independently confirmed.

The assault on the northeast of the capital was unleashed a day after opposition activists accused President Bashar al-Assad's regime of a gruesome new massacre in Daraya, southwest of the capital.

The Observatory said hundreds of bodies had been found in the small Sunni Muslim town after what activists described as brutal five-day onslaught of shelling, summary executions and house-to-house raids by government troops.

It said on Monday that a total of 334 bodies had now been found in Daraya.

Government troops launched the offensive on last Tuesday in a bid to crush insurgents who have regrouped in the southwestern outskirts of Damascus after the regime claimed to have retaken most of the capital late last month.

Assad vowed Sunday that he would not change course in the face of what he charged was a ‘conspiracy’ by Western and regional powers against Syria, which has been convulsed by 17 months of bloodshed.

"The Syrian people will not allow this conspiracy to achieve its objectives" and will defeat it ‘at any price,’ Assad said at a meeting with a top official from Iran, Syria's chief regional ally.

Assad, head of an Alawite-led regime in the majority Sunni Muslim country, has since March last year been using brutal force to smother a popular uprising that has descended into civil war.

Activists say around 25,000 people have been killed, while the United Nations is grappling with a major humanitarian crisis with over 200,000 refugees fleeing to neighbouring countries and 2.5 million people in need inside Syria.

Despite their far superior fire power, government forces are struggling to defeat rebels who have built strongholds in many parts of the country, particularly the northern city of Aleppo.

The Observatory said regime forces were also shelling Aleppo's Izaa district and the central city of Homs, reporting a death toll of 25 in violence nationwide on Monday.

Atrocities

Human rights groups have accused the regime of committing many atrocities during the conflict, and a UN panel said earlier this month it was guilty of crimes against humanity.

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

The Local Coordination Committees, a network of activists on the ground, said many victims had been summarily executed and their bodies burnt by pro-regime shabiha militias that have been transformed into a "killing machine".

"Bodies were found in fields, basements and shelters and in the streets," Observatory director Rami Abdel Rahman told AFP, adding that many of the victims had died in shelling or were summarily executed.

The reports cannot be independently verified because of severe restrictions on media operating in Syria.

Britain said that if confirmed, the Daraya massacre "would be an atrocity on a new scale."

State media said blamed the rebels for the killings and said Daraya had been was "purified of terrorist remnants."

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.

The Britain-based Observatory, which has a network of sources on the ground, reported a total of at least 149 people killed nationwide on Sunday.

August is already the deadliest single month of the conflict with at least 4,000 people killed, according to the Observatory.