At least 78 said killed in Syria's Hama province
Some witnesses claim victims include a 40 women and children, rebel groups call for investigation.
Activists said pro-government militia men and security forces killed at least 78 people, including children, in Syria's central province of Hama on Wednesday.
Some of those killed in the village of Mazraat al-Qabeer were stabbed to death, the activists said, and at least 12 bodies had been burned. Some activists who spoke to Reuters said at least 40 of the victims were women and children.
Syria's 15-month revolt against President Bashar Assad's rule has grown increasingly bloody in recent months, raising concerns the country may be slipping towards civil war.
The killings came less than two weeks after a massacre in the Syrian town of Houla, in which security forces and pro-Assad militia men known as 'shabbiha' killed 108 people, nearly half of them children.
Syrian forces had been shelling Mazraat al-Qabeer and the nearby village of Maazarif, which are around 20 km (12 miles) from the central city of Hama.
"Shabbiha headed into the area after the shelling and killed dozens of citizens, among them women and children," the Syrian Observatory for Human Rights said, describing a similar pattern of events as recounted from the Houla massacre on May 25.
Both massacres have happened in the presence of United Nations observers, a 300-strong force sent into Syria to observe a ceasefire deal brokered by international envoy Kofi Annan. The truce was hardly observed by the government or the rebels, who last week said they would no longer honor the ceasefire because of recent killings.
There was no comment from the government, and events on the ground are difficult to verify as Syria tightly restricts access to international media.
Activists called for an immediate investigation.
"The Syrian Observatory for Human rights calls on the international monitors to go immediately to the area. They should not wait to tomorrow to investigate this new massacre," the British-based Observatory 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."