President Bashar al-Assad has named a new prime minister to replace Syria's most senior government defector, as his forces continue to pound rebels in Aleppo.

Assad appointed Wael al-Halki, a Sunni Muslim from the southern province of Deraa, to head the government after Riyad Hijab fled on Monday after spending only two months in the job.

Hijab's dramatic escape across the border to Jordan dealt another blow to Assad's authority, already shaken by the assassination last month of four of his top security officials.

But Assad seems locked in a desperate contest with his mostly Sunni opponents, who are seeking to topple a system that's now dominated by members of the president's minority Alawite sect.

Assad is focused his fierce army counter-offensive on Syria's two main cities, reasserting control over much of Damascus before taking the fight to the northern commercial hub.

Rebels fighting in the Aleppo district of Salaheddine, a southern gateway to the city, said they had been forced to fall back from some frontline positions due to heavy government bombardment.

Rebel commander Abu Ali said: "There have been some withdrawals of Free Syrian Army fighters from Salaheddine".

As the battle for Aleppo raged, Assad's closest foreign backer Iran gathered ministers from like-minded states for talks about how to end the conflict.

Russia, another key ally of Assad, said its Ambassador to Tehran would attend the meeting.