Syrian rebels blew up a tunnel packed with explosives in the northern city of Aleppo on Friday, killing at least 20 pro-government fighters, activists and rebels said.
The Britain-based Syrian Observatory for Human Rights activist group said the blast took place near the Zahrawi market not far from the citadel in Old Aleppo. It said clashes followed the explosion.
A powerful rebel alliance called the Islamic Front claimed responsibility for the blast. It said in a tweet that it killed at least 40 government gunmen.
The Islamic Front also tweeted a video of the explosion. It shows a massive blast erupting from a skyline of rooftops and satellite dishes, throwing chunks of brick and a huge cloud of dust into the air.
The video appeared genuine and corresponded to other Associated Press reporting.
In early May, rebels also used bomb-packed tunnels to level a historic hotel in the Old City of Aleppo that was being used as an army base.
Such explosions have provided a reminder that the rebels, despite setbacks in other parts of the country, remain a potent force.