Syrian pro-government forces have killed at least 86 people in Hama province, many of them women and children, activists say.
The opposition said government-backed militia stabbed and shot their victims in the villages of Qubair and Maarzaf.
Syrian state TV said troops found some bodies after attacking "terrorists".
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.
UN monitors were able to confirm those deaths, and the BBC's Jim Muir in neighbouring Lebanon says activists say that the observers are likely to go to the Hama shortly to try to verify the latest claims.
Activist groups reported that Qubair and Maarzaf, about 20km (12 miles) north-west of the city of Hama, had come under heavy bombardment from security forces backed by tanks.
But they said much of the killing in Qubair 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.
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