More than 200 Syrians, mostly civilians, have been massacred in a village in the Hama region, after it was bombarded by helicopter gunships and tanks and then stormed by militiamen, opposition activists said.

If confirmed, it would be the worst single incident of violence in 16 months of conflict, which has seen rebels fight to topple President Bashar al-Assad.

The Revolution Leadership Council of Hama said the Sunni Muslim village of Taramseh was subjected 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 on Thursday evening.

Most of Assad's political and military establishment are minority Alawites, who form a branch of Shi'ite Islam. The revolt and the fighters behind it, and the street protesters who launched the revolt in March 2011, are mostly Sunni Muslims.