Syrian activists say rebels have blown up a tunnel they filled with explosives in the northern city of Aleppo, killing at least 20 pro-government fighters.
The British-based Syrian Observatory for Human Rights said the blast took place near the Zahrawi market in Old Aleppo.
Syria's largest rebel alliance, the Islamic Front, claimed responsibility for the attack. In a tweet, the group claimed it killed 40 government forces with the explosion.
The group also posted a video on Twitter that it said was of the blast. It shows a huge explosion throwing a massive cloud of debris into the air.
Rebels have recently been using tunnels to plant explosives beneath army positions.
Now in its fourth year, the Syrian conflict has killed more than 160,000 people.
Some information for this report comes from Reuters.