Fifteen Syrian regime troops, including two colonels, were killed in an ambush by rebels at dawn Wednesday in the northern province of Aleppo, the Syrian Observatory for Human Rights said.

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.