At least 13 troops loyal to the Syrian regime of Bashar al Assad died Tuesday in a double attack on the old police headquarters building in downtown Aleppo, the largest city in northern Syria, the Syrian Human Rights Observatory said.

The attack was carried out by two Islamist suicide bombers who detonated their bombs in two tunnels that had been dug below the building.

Meanwhile, near that site regime forces and rebel troops engaged in heavy fighting.

Syrian state-run television said that three explosions occurred at the police headquarters but there have been no reports of casualties as yet.

Bpmb attacks carried out from tunnels dug below government targets have become a frequent strategy of the Al Nusra Front, an affiliate of Al Qaeda in Syria, and other like-minded Islamist groups battling the Assad regime.

More than 171,000 people have been killed since the start of the Syrian conflict in mid-March 2011. EFE