Syrian activists have posted a video online that they say shows 15 bodies of men who were among dozens killed in heavy government shelling of a farming village in central Syria.

The Britain-based Syrian Observatory for Human Rights said it had reports that more than 150 killed were in Tremseh, although it had collected only 30 names of the victims.

Much remains unclear about the massacre which was reported late on Thursday. Another group, the Local Coordination Committees, said the dead numbered more than 200.

One resident, Laith al-Hamwi, said by phone on Thursday that government troops were shelling the village heavily and that dozens of people had been killed.

The activist claims could not be independently verified.

The latest report of violence came in the wake of the highest-level defector yet from President Bashar Assad's regime - his ambassador to Iraq.

Defections from the Syrian regime have stirred hopes in the West that Assad's inner circle will start abandoning him in greater numbers, hastening his downfall. But the tightly protected regime has largely held together over the course of the 16-month uprising, driven by a mixture of fear and loyalty.

The latest official to flee, Ambassador Nawaf Fares, announced that he was joining the revolution, asserting that only force will drive Assad from power.

"There is no road map ever with Bashar Assad, because any plan, any statement that is agreed on internationally he delays on and ignores," he said. "There is no way that he can be pushed from power without force, and the Syrian people realise this."

Syria's foreign ministry denounced Mr Fares, saying he should face "legal and disciplinary accountability".