“I condemn these atrocities in the strongest possible terms,” Annan said in a statement. There were no independent accounts of the number of dead or how they were killed. UN monitors in Syria are currently confined to Damascus because of mounting violence. An activist video — the only film record to appear so far— showed the bloodied corpses of 15 young men with faces or shirts drenched in blood. Most wore T-shirts and jeans. Syrian state television said there had been fighting in Tremseh and accused “armed terrorist groups” of committing a massacre there, but gave no death toll. It said three soldiers had been killed.

Opposition reports also said rebels of the Free Syrian Army had been killed in a battle. Lieutenant Ibrahim Zuaital-Tarkawai was among rebels who died “defending the people of Tremseh”, the Hama Revolution Leadership Council said.

“We can verify continuous fighting yesterday in the area of Tremseh,” said United Nations monitoring mission chief General Robert Mood.

“This involved mechanised units, indirect fire, as well as helicopters,” he said. UN monitors were ready to “go in and seek verification off acts if and when there is a credible ceasefire”, he said. Activists said the killing took place on Thursday, as the UN Security Council began negotiating a potentially crucialnew resolution on Syria. Washington said it showed the need to move to tougher action, but Russia again ruled out such a step.

The Sunni Muslim village, surrounded by farmland near the Orontes River, was first shelled then invaded by pro-government Alawite militiamen who swept in and killed victims one by one. Some civilians were killed while trying to flee, it said. Armed Assad loyalists known as Shabbiha have been accused repeatedly of cold-blooded indiscriminate killings carried out on the coattails of army offensives into rebel-held districts.

Therevolt and the fighters behind it, and the street protesters who launched the revolt in March 2011, are mostly Sunni Muslims. The insurgents cannot match the Syrian army’s firepower but they have established footholds in villages, towns and even cities across Syria, coming under attack from Assad’s forces to respond fiercely with helicopter gunships and artillery. Following high-profile defections of a family friend and atop diplomat, both Sunnis, analysts said cracks were appearing in Assad’s Alawite-dominated rule. 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 a tough new UN resolution. Britain, the United States, France and Germany want to 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. Russia’s Deputy UN Ambassador Alexander Pankin warned Moscow would use its veto if it had to. “We are definitely against Chapter 7,” he said. “Anything can be negotiated, but wedo not negotiate this, this is a red line.”