Opposition forces in Syria have detonated bombs in tunnels under Aleppo and killed at least 13 loyalists of President Bashar al-Assad in the northern city, activists and a monitoring group say.
The bombs were reportedly placed in three tunnels running under historic parts of the city, including near the governorate building.
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"The attack is meant to break the government's first defence line to Aleppo Castle [in the centre of the city] and storm the area," Adel, a member of the Islamic Front, a coalition of rebel groups that claimed responsibility for the attack on Tuesday night, told Al Jazeera.
Fighting has been raging in the area for days in the commercial heart of Aleppo, an area whose districts are divided between rebel and government control.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said at least 13 government soldiers and other forces fighting alongside them were killed.
The Islamic Front put the death toll at 57 troops loyal to Assad.
Syria's state news agency made no mention of the attack.
Aleppo is one of Syria's main contested cities and government forces took strategic ground around it in June, squeezing the main rebel supply line into the city after months of battlefield gains by the Syrian government.
The government advance was reportedly backed by fighters from the Lebanese Shia armed group Hezbollah, an ally of Assad's regime.
Rebels advanced into Aleppo, once Syria's commercial hub, in 2012 from the north and took districts in the centre of the city.
The army has since mainly held the west and south of Aleppo, but has been unable to push out opposition fighters.
Forces loyal to Assad now control most of Damascus, along with the main highway from the capital through to Homs and the western Mediterranean coast.
Opposition fighters, including the powerful al-Qaeda offshoot Islamic State, control much of the north and east.
Source: Al Jazeera and agencies