An air strike near Syria's battleground second city Aleppo killed the military leader of the largest rebel alliance and at least one other commander, jihadist sources and a monitor said.

The strike on a meeting of commanders of the Army of Conquest alliance came hot on the heels of a major defeat for the rebels, which saw them under renewed siege inside Aleppo after an army advance this week. 

Former Al-Qaeda affiliate Al-Nusra Front, renamed Fateh al-Sham Front when it broke ties in July, announced on Twitter "the martyrdom" of commander Abu Omar Saraqeb in an air strike.

The jihadist Fateh al-Sham is a leading member of the Army of Conquest which groups its fighters with Islamist factions like Ahrar al-Sham in the most prominent anti-regime alliance. 

The Syrian Observatory for Human Rights said unidentified aircraft hit the Army of Conquest meeting on Thursday night, killing Saraqeb and another rebel commander named as Abu Muslim al-Shami.

Observatory head Rami Abdel Rahman said it was not immediately clear whether the strike was carried out by the US-led coalition, the Syrian regime or its Russian ally -- all of whom are conducting bombing raids in northern Syria. 

According to the Observatory, Abu Omar Saraqeb was a leading member of Al-Qaeda in Iraq in its fight against the US-led occupation after 2003 before becoming a key commander of Al-Nusra Front and then Fateh al-Sham in Syria. 

He reportedly also founded Al-Nusra Front's Lebanon branch which has claimed responsibility for several bombings in Syria's western neighbour.

He operates under different noms de guerre, making it difficult to know his nationality.

He led a successful offensive by the Army of Conquest in spring last year which saw it seize control of nearly all of the northwestern province of Idlib.

But the alliance has been less successful in and around Aleppo, where it was dealt a major blow by regime forces this week.

In early August, Saraqeb led an offensive against pro-government fighters besieging the rebel-held east of the divided city and opened up a new supply route from the south that broke the siege.

But this week regime loyalists recaptured nearly all of the territory taken last month and reimposed the blockade. 

Jihadism expert Charles Lister said Saraqeb had helped found the the Army of Conquest last year and once served as the "emir" of Idlib province.

Amid Syria's fractious rebel movement, "he's seen widely as an Islamist unifier," Lister wrote. 

Surce: AFP