Fierce clashes erupted late on Saturday as Syrian government forces and their allies battled their opponents including al Qaeda fighters south of Aleppo.

The Syrian Observatory for Human Rights reported anti-Assad forces advancing and taking over two hilltops.

The Britain-based monitoring group’s director Rami Abdulrahman said this might mark a collapse in the area of a fragile cessation of hostilities agreement which has been in place for several weeks and slowed fighting but not halted it.

Fighting had already began between the government forces and Assad opposers including those from the al Qaeda-affiliated Nusra Front nearby and southwest of Syria’s former commercial hub Aleppo, the Observatory said.

Syrian and Russian warplanes launched dozens of air strikes in the area, it said.

The Russian- and U.S.-brokered ceasefire agreement excludes operations against al Qaeda and ISIS.

Fighting has continued against ISIS, and between Syrian government forces and other groups fighting Assad in some areas in the west of the country.

Warring sides in the conflict have traded blame for violations of the truce.