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."