Syrian troops with tanks and helicopters slaughtered more than 150 people in a central village, rights activists said on Friday, prompting the opposition to call for urgent UN intervention.
Reports of the massacre came after UN Security Council ambassadors held their first talks on rival Russian and Western draft resolutions on Syria, with Moscow spurning calls for sanctions against President Bashar al-Assad's regime.
No progress was reported, with a July 20 deadline looming. That is the end date of the mandate for the UN mission to the conflict-stricken country, where activists say more than 17,000 people have died since March 2011.
The Syrian Observatory for Human Rights said government troops had on Thursday massacred more than 150 people in Treimsa village, while a rebel leader put the toll at more than 200.
If confirmed, the killing at Treimsa in the central province of Hama would rival the massacre at Houla on May 25, when a pro-Assad militia and government forces were accused of killing at least 108 people.
Observatory chief Rami Abdel Rahman told AFP by telephone that the bodies of 30 villagers had already been identified following the sustained attack, which brought Thursday's total death toll in the conflict-torn nation to well over 200.
"Some are estimating higher numbers, but even at around 150, especially considering how small the town is, this might be the biggest massacre committed in Syria since the start of the revolution," he said.
"The army must have got the green light to commit a massacre of this scale, and I bear President Bashar al-Assad responsible for the killing."
The Syrian National Council, the main opposition alliance, urged the UN Security Council to pass a binding resolution against Assad's regime.
"To stop this bloody madness which threatens the entity of Syria, as well as peace and the security in the region and in the world, requires an urgent and sharp resolution of the Security Council under Chapter VII (of the UN Charter) which protects the Syrian people," said.
Chapter VII allows for punitive measures against regimes considered a threat to the peace, including economic sanctions and military intervention.
Syria's Muslim Brotherhood said Annan and Syrian allies Iran and Russia must through their inaction shoulder the blame for the killing reportedly by al-Assad's forces of at least 150 villagers.
"We don't consider the monster Bashar as being solely responsible for this heinous crime ... but (also) Kofi Annan, the Russians and the Iranians and all countries which pretend to be guardians of peace and stability in the world but who remain silent," the Brotherhood said in a statement.
Syria meanwhile accused "bloodthirsty media" and "terrorist gangs" of being behind the massacre.
"The bloodthirsty media in collaboration with gangs of armed terrorists massacred residents of Treimsa village ... to sway public opinion against Syria and its people and provoke international intervention on the eve of a UN Security Council meeting," state-run news agency SANA said.
Rebel leader Abu Mohamad, chief of a group based further to the north of Treimsa, said the attack using helicopters, tanks and multiple rocket-launchers had killed more than 200 people in the village.
Abu Mohamad said he had been in phone contact with a resident of Treimsa who told him that government forces were on hills a few kilometres (miles) outside the town.
The army and the shabiha, pro-regime militia who are said to accompany troops to make sure they do not desert, started to bombard Treimsa Thursday around 11:00 am (0800 GMT) and finished around 9:00 pm, according to Abu Mohamad.
But a Hama-based activist who identified himself as Abu Ghazi said via Skype that regime troops started shelling the village earlier, at around 6:00 am.
"That was followed by clashes with the (rebel) Free Syrian Army, but the FSA does not have a big presence in Treimsa and could not fight long," said the activist.
"The number of martyrs is very high partly because the army shelled a mosque where scores of people had taken shelter, to treat the wounded and hide from the bombs."
The village, which had a population of 7,000, he said, "is empty now. Everyone is dead or has run away."
"Almost 30 army vehicles arrived, and surrounded the village completely. There wasn't a single way out," said Ibrahim, another activist from Treimsa. "Anyone who tried to escape through the fields was shot."
Pro-regime militiamen from neighbouring Alawite villages entered Treimsa after the army raided it, Ibrahim said via Skype. "After the shelling, the army came in with light weapons, and the shabiha followed, armed with knives."
Clashes inside the besieged village were vicious, he said, noting that "whole families were killed. There was a real street war for several hours."
Treimsa is located near Qubeir, where at least 55 people were killed on July 6, according to the Observatory. Like Qubeir, Treimsa is a majority Sunni town situated near Alawite villages.
President Assad belongs to the Alawite community -- an offshoot of Shiite Islam -- although the vast majority of Syrians are Sunni.
Protests broke out across Syria to protest the latest killings in Treimsa, according to amateur videos posted on YouTube by activists.
In New York, the UN Security Council wrangled over a resolution on the future of the troubled UN mission in Syria.
Russia and the Western powers have proposed rival resolutions on renewing the UN mission in Syria and securing the implementation of the peace plan of UN-Arab League envoy Kofi Annan.
The Security Council must hold a vote before the mandate of the UN Supervision Mission in Syria, or UNSMIS, runs out next week.
In a related development, Annan will on Monday meet Russian Foreign Minister Sergei Lavrov for talks on the crisis in Syria, Russian news agencies reported.
"The talks will focus on the current situation in Syria and the chances of an internal Syrian solution," a Russian foreign ministry source, who was not named, told the ITAR-TASS news agency.
The talks would most likely touch on a UN resolution that would extend the mandate of the UN mission in Syria, said the source.
Annan previously visited Moscow in March for talks with Lavrov and Russian leaders, but his peace plan to end the conflict between the regime and the opposition has made little headway in the last months.