BEIRUT – 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. Clashes between rebels, belonging to Islamic groups, including the al-Qaida-linked Nusra Front, and Assad’s forces broke out after the blasts and fighting raged into the night, the Observatory said. The group has been documenting Syrian conflict since it started in March 2011 through a network of activists on the ground. Another opposition group, the Syria-based Local Coordination Committees, also reported the Aleppo blasts. It said there were an unknown number of casualties on the government side. There was no immediate claim of responsibility for the attack that comes amid reports of a surge in deadly attacks by the al-Qaida-breakaway Islamic State group targeting Assad’s forces. More than 2,000 Syrians – almost half of them pro-government forces – have been killed in just over two weeks, marking one of the worst death tolls in the country’s civil war.