Air strikes targeting rebel-held districts of Aleppo intensified on Tuesday, after a lull of several days which the Syrian army said was designed to allow civilians to leave.

The city has been divided between government and rebel control for years. "The bombing has been very intense," Selmo told Reuters on Thursday, speaking from Syria in an area outside Aleppo city.

The Syrian army launched an assault to take rebel-held areas of Aleppo last month with Russian air support and Iranian-backed militias.