Insurgents fighting in Syria to oust President Bashar al-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 pro-opposition monitoring group 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 probably housed troops.

Clashes between rebel groups, including the Al-Qaeda-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 using a network of activists on the ground to document the Syrian conflict since it started in March 2011.

Another opposition group, the Syria-based Local Coordination Committees, also reported the Aleppo blasts. It said there was an unknown number of casualties on the government side.

There was no immediate claim of responsibility for the attack, which comes amid reports of a surge in deadly attacks on Assad’s forces by the Al-Qaeda splinter group Islamic State, formerly known as ISIL.

More than 2,000 Syrians — almost half of them among pro-government forces — have been killed in just over two weeks, marking one of the worst death tolls in the country's civil war.

The Islamic State group has recently taken swaths of territory in eastern Syria and whole cities and towns in neighboring Iraq. It merged the territories last month and declared an Islamic state.

Increased targeting of Assad's forces in northern Syria could signal shifting priorities for Sunni militants, seeking to consolidate their hold on territory and resources along the border with Turkey.

The attacks are also a powerful reminder that the rebels can still deal a heavy blow to Assad's forces in the heart of Aleppo's urban center, one of the biggest prizes in the civil war. Regaining control of Aleppo would boost Assad's confidence after his forces retook territory from the opposition in central Syria, and along the border with Lebanon and around Damascus in time for its June 3 presidential election.

Assad won a third, seven-year term in a vote that was dismissed by the West and the opposition as a sham.

The Syrian conflict started in March 2011 as a largely peaceful uprising against Assad's rule. It has turned into an armed revolt after some opposition supporters took up arms to fight a brutal government crackdown. It gradually became a civil war, pitting predominantly Sunni rebels against Assad's government that is mostly made up of Alawites, a sect in Shia Islam.

More than 170,000 people have been killed in the fighting, and nearly a third of Syria's 23 million inhabitants have been uprooted from their homes.

The Associated Press