BEIRUT - The commander of Syria's largest rebel coalition has been killed in an air strike near Aleppo, in what analysts say is the biggest blow to the alliance since its formation.

The strike on a meeting of leaders of the Army of Conquest came after a major defeat for the rebels, which saw them once again besieged inside Aleppo after a pro-government 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 alliance, which groups its fighters with those of Islamist factions like Ahrar al-Sham.

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.

"This is undoubtedly the biggest blow ever inflicted on Fateh al-Sham and the Army of Conquest in Syria," Abdel Rahman told AFP.

According to the Observatory, Saraqeb was a leading member of Al-Qaeda in Iraq - a precursor to the Islamic State group - in its fight against the US-led occupation after 2003.

He went on to become a key commander of Al-Nusra Front in Syria.

Abu Omar 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 major 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 encircling 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 backed by Russian warplanes recaptured nearly all of the territory taken last month and reimposed the blockade on the estimated 250,000 civilians living in rebel-held neighbourhoods.

Pro-jihadist accounts on Twitter mourned Saraqeb's death, calling him a "heroic martyr."

"The targeting of the symbols of this blessed revolution will only increase our determination to achieve our goals," pledged one brigade of the Army of Conquest.

Charles Lister, a senior fellow at the Middle East Institute think-tank, said Saraqeb had helped found the Army of Conquest last year and once served as the "emir" of Idlib province.

Amid Syria's fractious rebel movement, he was an "advocate of military unity" as well as "an ideological hardliner who held strong views on how society should be controlled," Lister told AFP.

He said the unidentified raid on Thursday targeted a meeting that would plan a fresh rebel offensive "to break the re-besieging of Aleppo."

It remains unclear the extent to which Abu Omar's death will impact the fight for Aleppo and the broader rebel movement in Syria.

According to Syria expert Thomas Pierret, the Army of Conquest remains "the only group that could credibly claim that it can break the siege of Aleppo."

"The consequences of this assassination will undoubtedly be more political than military, as jihadist groups are generally well-prepared to replenish their cadres," he told AFP.

Aleppo has been ravaged by fighting since the rebels seized eastern districts in 2012, with the former commercial hub transformed into a bombed-out city.