Nearly 40 pro-regime fighters were killed Thursday (July 21st) when opposition fighters blew up a tunnel under a regime position in Syria's Aleppo city, AFP reported Friday.

The Syrian Observatory for Human Rights said 38 members of the regime forces were killed in the blast, which brought down a building in Aleppo's Old City.

A video posted online by the Thuwwar al-Sham opposition group purported to show the incident, with members of the force walking though a long tunnel and preparing barrels full of explosives, followed by a massive blast leveling a multi-story building, filmed from multiple angles.

Also on Thursday, regime bombardment hit two opposition-held neighbourhoods in east Aleppo city, killing 15 people, among them six children, the Observatory said.

Meanwhile, the UN on Thursday called for a weekly 48-hour truce in Aleppo to allow aid deliveries to reach some 250,000 civilians facing starvation.

The head of the UN-backed humanitarian task force for Syria, Jan Egeland, said aid agencies were ready to send supplies to the city's opposition-held eastern districts but raging violence has blocked convoys from deploying.

"The clock is ticking," he said, describing Aleppo residents as being "on the brink of starvation".