Syrian government forces have recaptured parts of Aleppo city which were lost to rebels last month, placing rebel-held districts in the city's east once again under siege, monitors said.
Reports said government troops had recaptured two military academy sites in the south of the city and severed a recently-established rebel supply line.
The government offensive was backed by Syrian and Russian air strikes.
There are about 250,000 civilians living in the city's rebel-held areas.
UK-based monitoring group the Syrian Observatory for Human Rights confirmed that government troops had retaken the Armaments Academy and Air Force Technical College.