More than 200 Syrians, mostly civilians, were massacred in a village in the rebellious Hama region when it was blasted by helicopters and tanks and then stormed by Assad regime militia, opposition activists said Thursday.

If confirmed, it would be the worst single incident of violence in 16 months of conflict in which rebels are fighting to topple President Bashar al-Assad and diplomacy to halt the bloodshed has been stymied by jostling between world powers.

The Revolution Leadership Council of Hama told Reuters the Sunni Muslim village of Taramseh was subjected Thursday to a barrage of heavy weapons fire before pro-government Alawite militiamen swept in and killed victims one by one.

More than 220 people fell today in Taramseh. They died from bombardment by tanks and helicopters, artillery shelling and summary executions, the regional opposition group said in a statement Thursday evening.

Syrian state television said three security personnel had been killed in fighting in Taramseh and it accused armed terrorist groups of committing a massacre there.

Fadi Sameh, an opposition activist from Taramseh, told Reuters he had left the town before the reported killing spree 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 many of the bodies were taken to a local mosque.

Ahmed, another local activist, told Reuters: 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 limit access for independent journalists.

Seventy-eight people were shot or stabbed dead or burned alive in the village of Mazraat al-Qubeir, a Sunni hamlet, by fighters of Assad's Alawite sect on June 6, and 108 men, women and children were massacred in the town of Houla on May 25.

Earlier on Thursday, the first ambassador to abandon Assad called on the army to turn your guns on the criminals of the government as troops backed by tanks swarmed into a suburb of Damascus on Thursday to flush out rebels.

Nawaf al-Fares, a Sunni Muslim who has close ties to the security services, was Syria's ambassador to its neighbor Iraq, one of its few friends in the region.

Coming just days after the desertion of Manaf Tlas, a Sunni brigadier general in the elite Republican Guard who grew up with the president, his defection gave the anti-Assad uprising one of its biggest political lifts.

But Assad's strongest strategic ally, Russia, stuck by him on Thursday with a clear warning to his Western and Arab enemies that it would not even consider calls for a tough new resolution by the U.N. Security Council in New York.