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Syrian pro-government forces have killed 78 people in a single village in Hama province, many of them women and children, activists say.
The opposition said government-backed militia stabbed and shot their victims in the village of Qubair.
Damascus denied a massacre, saying "terrorists" had killed nine people.
Neither account could be confirmed, but activists said 140 had been killed nationwide on Wednesday - one of the bloodiest days of the uprising.
It comes less than two weeks after 108 people were killed in a massacre in Houla.
Analysts say the continuing unrest suggests diplomatic efforts to resolve the crisis are having little impact on the ground.
On Thursday, UN-Arab League envoy Kofi Annan is expected to urge the UN Security Council to create a new contact group to help end the violence.
UK Prime Minister David Cameron condemned the "brutal and sickening" killing of civilians and called for concerted international action.
'Heinous crime'
Security forces bombarded Qubair, a village of fewer than 30 houses, about 20km (12 miles) north-west of the city of Hama, according to activists.
But they said much of the killing was done by accompanying groups of pro-government militiamen known as shabiha, who had come from nearby pro-government villages.
The activists said they shot at close range and stabbed many people, and that some of the bodies were later burnt in houses that were set on fire.
"They executed [nearly] every person in the village. Very few numbers could flee. The majority were slaughtered with knives and in a horrible and ugly way," one activist in Hama told the BBC's World Tonight.
"The small number of villagers who fled were the only people remaining who could tell the world about this horrible massacre."
One of Qubair's residents told the BBC that when the army and militia left the village, he had discovered about 40 bodies - mostly women and children who had been stabbed to death.
Among the victims were four members of his family, the villager said. He added that he saw the burned corpse of a three-month-old baby.
The Local Co-ordination Committees, an activist network, said 78 people had died in Qubair, including 35 members of one family.
However, Syrian officials said reports of a massacre were "completely false".
State TV reported that security forces had launched an attack on an "armed terrorist stronghold" in Qubair after appeals from citizens.
Troops came across the bodies of two women and a number of children, bound hand and foot.
In a statement on state TV quoted by AFP news agency, the government said that "a terrorist group has committed a heinous crime".
The LCC said the Qubair killings had brought the total number of people killed nationwide by security forces on Wednesday to 140.
The BBC's Paul Danahar, in Damascus, says UN monitors are on their way to the village to investigate the killings.
The 297 unarmed observers are in Syria to verify the implementation of a peace plan negotiated by Mr Annan, which includes a ceasefire that supposedly came into force in mid-April.
Mr Annan is now thought to want a contact group including permanent members of the Security Council and key regional powers.
But the US Secretary of State, Hillary Clinton, has already said it is unthinkable to involve in the process a country like Iran, which - she said - was state managing the Syrian government's assault on its own people.
Meanwhile at a regional forum in Beijing, China and Russia have reiterated their opposition to any military intervention or attempts at regime change.
