The Britain-based watchdog said the ambush took place near the village of Al-Rai, scene of clashes between opposition fighters and regime forces.

It said two rebel fighters were also killed.

The killings took place in spite of a putative ceasefire that came into effect on April 12, which the United Nations has accused both sides of violating.

The truce is part of a peace plan brokered by United Nations-Arab League envoy Kofi Annan.

The plan calls for a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate and the release of detainees.

According to the UN, more than 9,000 people have been killed in Syria since an anti-regime uprising broke out in March last year. (AFP)