Syrian rebels have detonated explosive -packed tunnels under buildings in the northern city of Aleppo, killing at least 13 pro-government troops, and destroying part of the old city, opposition activists have said.

Syria's main opposition group the Islamic Front posted footage of the explosions on YouTube shortly after the bombs were detonated on Tuesday.

The UK-based Syrian Observatory for Human Rights said rebels blew up two tunnels, dug near the ancient quarter of Aleppo that has been the site of some of the fiercest fighting in the Syrian conflict.

The Observatory said the blasts killed at least 13 soldiers and pro-government fighters. It said one bomb went off under a police building that likely housed troops.

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.

Al Jazeera cannot independently verify the information.

Members of the Islamic Front earlier this year blew up a Syrian army barracks based in a hotel near the old citadel, after digging tunnels underneath and then packing them with explosives.

Clashes between rebels belonging to groups including the al-Qaeda-linked Nusra Front broke out with Assad's forces after the blasts, and fighting flared into the night, the Observatory said.

Thousands die in fortnight

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

The Islamic State group has recently taken large swaths of territory in eastern Syria and whole cities and towns in neighbouring 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 fighters seeking to consolidate their hold on territory and resources along the border with Turkey.

But 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 early June.