WARNING, STORY CONTAINS GRAPHIC IMAGES OF DEATH
More than 200 people, most of them civilians, have been killed in shelling and other attacks by regime forces in Syria's central Homs province, activists reported.
Those who survived the shelling were later shot in the head, "execution style" by troops on the ground in the village of Taramseh, a statement from the Hama Revolutionary Council said.
Syrian TV said "armed terrorist groups" took part in the massacre. Fadi Sameh, an opposition activist from Taramseh left the district before the attacks, but was in touch with residents.
He told Reuters: "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."
Syrian activists have posted a video online purporting to show casualties of the massacre.
The unverified video features a line of dead bodies, including what appears to be a woman clasping a baby in her arms.
Scroll down for more video stills. WARNING: GRAPHIC CONTENT
There were few other details on the attack, which was also reported by the Local Co-ordination Committees activist group and the Britain-based Syrian Observatory for Human Rights.
Reports of the incident have not be independently verified, but if they are accurate, it may represent the single biggest massacre since the uprising began.
Activists say more than 17,000 people have been killed since the uprising against President Bashar Assad began in March 2011, and he is coming under growing international pressure to stop the violence. But as the bloodshed continues, and the conflict morphs into an armed insurgency, hopes for a peaceful transition are dimming.
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".
In Washington, state department spokesman Patrick Ventrell hailed what he called the "first major diplomatic defection", adding: "We think this a wider sign that the regime is feeling the pressure. The pressure is up and the regime is really starting to fall apart."