Nearly 40 Syrian soldiers and pro-regime fighters were killed when rebels blew up a tunnel under a government position in Aleppo city, a monitor said Friday.
The Britain-based Syrian Observatory for Human Rights said 38 members of government forces were killed in Thursday's blast, which brought down a building used by the regime in Aleppo's Old City.
The monitor had earlier reported a toll of 14.
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A video posted online by the Thuwwar al-Sham rebel group purported to show the incident, with members of the force walking though a long tunnel and preparing barrels full of explosives.
"We are now inside the tunnel that will be detonated soon, God willing, the tunnel under the traffic branch building, which is an important headquarters for the Assad regime and its mercenaries," a rebel says in the video.
The footage then shows a massive blast levelling a multi-storey building, filmed from multiple angles.
A huge geyser of dirt and smoke shoots upwards from the scene of the blast, after which gunfire can be heard.
Aleppo was once Syria's economic powerhouse, but it has been ravaged by the conflict that began in March 2011.
The city has been roughly divided between government control in the west and rebel control in the east since shortly after fighting began there in mid-2012.
The east of the city has been under siege for the past two weeks, since government forces severed the only remaining supply route into rebel-held districts.
The government advance has raised fears for more than 200,000 people who remain in the east of the city, where food shortages and spiralling prices have already been reported.