Syrian government forces today killed more than 200 people, most of them civilians, in a village in the province of Hama, opposition activists said.
The village of Taramseh was shelled by Syrian troops and later stormed by pro-government Shabbiha militia, they said.
Several people in the village were killed by the shelling and more were shot later in the head, execution-style, they added.
Syrian television said three security personnel had been killed in fighting in Taramseh, and accused "armed terrorist groups" of committing a massacre there.
A statement by the Hama Revolutionary Council said: "More than 220 people fell today in Taramseh.
“They died from bombardment by tanks and helicopters, artillery shelling and summary executions."
Fadi Sameh, an opposition activist from Taramseh, said he had left the town before the reported massacre but was in touch with residents.
"It appears that Alawite militiamen from surrounding villages descended on Taramseh after its rebel defenders pulled out, and started killing the people. Whole houses have been destroyed and burned from the shelling.
"Every family in the town seems to have members killed. We have names of men, women and children from countless families," he said, adding that many of the bodies were taken to a local mosque.
Ahmed, an activist from the Union of Hama Revolutionaries said: "We have reports of more than 220 killed. So far, we have 20 victims recorded with names and 60 bodies at a mosque.
“There are more bodies in the fields, bodies in the rivers and in houses ... people were trying to flee from the time the shelling started and whole families were killed trying to escape."
The reports could not be independently confirmed. Syrian authorities severely restrict the activities of independent journalists.
If the accounts are accurate, the incident would be the worst of its kind in the rebellion against the rule of President Bashar al-Assad that began 16 months ago.
Former ambassador calls on army to abandon Assad
Earlier today, the first ambassador to abandon Syrian President Bashar al-Assad called on the army to "turn your guns on the criminals" of the government.
His statement came as troops backed by tanks moved into a suburb of Damascus to flush out rebels.
Nawaf al-Fares, who has close ties to the security services, was Syria's ambassador to its neighbour Iraq, one of its few allies in the region.
Syria's strongest ally, Russia, stuck by President Assad with a clear warning to his Western and Arab enemies that it would not even consider calls for a tough new resolution by the UN Security Council in New York.
Britain circulated a draft yesterday, backed by the United States, France and Germany, that would make compliance with a transition plan drafted by international envoy Kofi Annan enforceable under Chapter 7 of the UN Charter.
This would allow the council to authorise actions ranging from diplomatic and economic sanctions to military intervention.
But as council members began negotiations on a resolution to renew the UN Syria monitoring mission, Russia's Deputy UN Ambassador Alexander Pankin made clear Moscow would use its veto if it had to.
"We are definitely against Chapter 7," he said. "Anything can be negotiated, but we do not negotiate this, this is a red line."
Mr Annan himself asked the 15-member council to agree on "clear consequences" if the Syrian government or opposition failed to comply with his plan, which has produced neither a ceasefire nor political dialogue since it was agreed in April.
The British draft threatens the Syrian government with sanctions unless it stops using heavy weapons and withdraws its troops from towns and cities within 10 days.
While the insurgents cannot match the Syrian army's firepower, they have managed to establish footholds in towns, cities and villages across Syria, often prompting Mr Assad's forces to respond with helicopter gunships and artillery.