Assad appointed Wael al-Halki, a Sunni Muslim from the southern province of Deraa where the Syrian uprising erupted 17 months ago, to head the government after Riyad Hijab fled on Monday after spending only two months in the job.

Hijab's 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 and rebel gains in Damascus, Aleppo and swathes of rural Syria.

But Assad, grimly shrugging off such setbacks, seems locked in a desperate contest with his mostly Sunni opponents seeking to end half a century of Baathist rule and topple a system now dominated by members of the president's minority Alawite sect.

Assad has 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 on Thursday by withering bombardment which had reduced buildings to rubble.

"There have been some withdrawals of Free Syrian Army fighters from Salaheddine," rebel commander Abu Ali said, adding that rebels were regrouping for a counter-attack.

Another combatant said at least 30 people had been killed in Salaheddine, where fighting has ebbed and flowed for two days.