At least 86 people, including many women and children, have been killed by Syrian pro-government forces in Hama province, opposition activists say.
More than 20 children and 20 women were reportedly among those who died in the villages of Qubair and Maarzaf, in what the activists called a "massacre".
Syrian State TV said troops found some bodies after attacking "terrorists".
Neither account could be confirmed, but it comes less than two weeks after 108 people were killed in Houla.
Witnesses blamed pro-government militiamen, while the government of President Bashar al-Assad accused "armed groups" seeking to trigger foreign military intervention.
'Heinous crime'On Wednesday evening, 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.
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The activists said they shot at close range and stabbed many people, including women and children under the age of two, and that some of the bodies were later burnt in houses which were set on fire. Others were taken away by the shabiha, they added.
"They executed [nearly] every person in the village. Very few numbers could flee. They majority were slaughtered with knives and in a horrible and ugly way," one activist in Hama told the BBC's World Tonight.
"[They] are failing to save the lives of the wounded because they are very poor people - usually they are Bedouin who... have no kind of healthcare," he added.
"The small number of villagers who fled were the only people remaining who could tell the world about this horrible massacre."
The Local Co-ordination Committees, an activist network, said 78 people had died in Qubair, including 35 members of one family.
Later on Wednesday, Syrian state TV quoted an official source as saying that security forces, responding to appeals from citizens, had launched an attack on an "armed terrorist stronghold" in Qubair.
The security forces came across the bodies of two women and a number of children, bound hand and foot, in the village, who the coroner said had been killed at 10:00, when the "armed terrorist groups" were still in the village, the official added.
In a statement on state TV quoted by AFP news agency, the government said that "a terrorist group has committed a heinous crime".
"The reports by the media are contributing to spilling the blood of Syrians," the statement added.
The LCC said the Qubair killings had brought the total number of people killed nationwide by security forces on Wednesday to 140.
Annan appealThe BBC's Jim Muir in Beirut says there has been no independent confirmation of these latest reports and no video has surfaced on the internet to back them up.
But the news of the Houla massacre emerged in a similar way and the details given by activists were later confirmed by UN ceasefire observers on the ground, our correspondent adds.
Annan's six-point peace plan
1. Syrian-led political process to address the aspirations and concerns of the Syrian people
2. UN-supervised cessation of armed violence in all its forms by all parties to protect civilians
3. All parties to ensure provision of humanitarian assistance to all areas affected by the fighting, and implement a daily two-hour humanitarian pause
4. Authorities to intensify the pace and scale of release of arbitrarily detained persons
5. Authorities to ensure freedom of movement throughout the country for journalists
6. Authorities to respect freedom of association and the right to demonstrate peacefully
The UK-based Syrian Observatory for Human Rights called on monitors to go immediately.
"They should not wait to tomorrow to investigate this new massacre," it said in a statement. "They should not give the excuse that their mission is only to observe the ceasefire, because many massacres have been committed during their presence in Syria."
The 297 unarmed military observers are in Syria to verify the implementation of the peace plan negotiated by the UN and Arab League envoy, Kofi Annan, including a ceasefire that supposedly came into force in mid-April.
On Thursday, Mr Annan will urge the UN Security Council to create a new contact group to help end the violence, diplomats say. It will include permanent members of the council, and key regional powers.
The BBC's Nada Tawfik at UN headquarters in New York says Mr Annan hopes involving countries like Iran and Turkey will speed up efforts to start a political transition in Syria that have stalled under his plan.
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.
And speaking at a meeting of the "Friends of Syria" countries in Istanbul on Wednesday, UK Foreign Secretary William Hague Iran's involvement would make any contact group "unworkable".
