Syrian troops and their allies, backed by Russian jets, attacked insurgent fighters south of Aleppo on Friday, a source in Syria and a monitoring group said, as the army expands a counter-offensive with support from Moscow's two-week-old air campaign.

"The military operation has started in rural southern Aleppo," a pro-government source in Syria said.

Rami Abdulrahman, director of the Syrian Observatory for Human Rights, said there were heavy clashes near the Jebel Azzan region, about 12 km (8 miles) south of Aleppo city, control of which is divided between government forces and rebels.

The area the army and Russian jets were targeting was close to a main road heading south towards the capital Damascus, Abdulrahman said.

Since Russia launched air strikes on Sept. 30 in support of President Bashar al-Assad, the army has waged offensives against insurgent-held regions in western Syria, including a rebel pocket north of Homs and areas of Hama, Idlib and Latakia provinces taken by the rebels over the summer.