BEIRUT (AP) — Insurgents fighting in Syria to oust President Bashar Assad detonated bomb-packed tunnels under buildings in the contested northern city of Aleppo on Tuesday, killing at least 13 pro-government troops, opposition activists said.

The Britain-based Syrian Observatory for Human Rights said rebels detonated explosives in two tunnels, dug under the ancient quarter of Aleppo that has been the site of some of the fiercest fighting in the Syrian conflict, now in its fourth year. The Observatory said the blasts killed at least 13 soldiers and pro-government militiamen late Tuesday. It said one bomb went off under a police station that likely housed troops.