Rebel groups led by two of the most powerful Syrian jihadi forces opened an offensive against Syrian army and Iran-backed allied militia in the southwestern parts of Aleppo on Sunday in a bid to reverse a government siege of opposition-held eastern Aleppo.

The influential Salafist group Ahrar al-Sham and its jihadi ally Levant Conquest Front, formerly known as the al-Qaeda linked Nusra Front before a rebranding last week, were reported to have taken positions held by a combination of Syrian regime forces, Iranian militia and Hezbollah in the south and southwest of Aleppo.

The Syrian army and allied militia backed by Russian airpower in mid-July essentially cut off and surrounded rebel-held eastern Aleppo, where tens of thousands of civilians still live. It was a major blow to the opposition, which has controlled the eastern part of the city since 2012.

The Syrian army confirmed a rebel offensive on Sunday but said it had pushed back insurgent forces from key strategic points.

The UK-based Syrian Observatory for Human Rights, which monitors fighting, said the rebels would be up against an array of forces.

"It will be a long and difficult battle," said Observatory chief Rami Abdurrahman. "The army is supported by a large number of Iranians and fighters from Hezbollah, not to mention the Russian planes."

Seizing all of Aleppo, Syria's largest city before the war, would be a major victory for Syrian President Bashar al-Assad. For rebels, re-opening a lifeline in the city would keep the rebellion alive.

Yet the role of the Levant Conquest Front also raises further questions about the jihadi group's relationship with other rebel factions.

Even after distancing itself from al-Qaeda last week, the rebranding is considered by many analysts and the United States as nothing more than an effort to paper over ideological and operational links between the Levant Conquest Front and the global jihadi movement.

The US has been pressuring rebel groups to distance themselves from the former al-Nusra Front, while at the same time discussing with Russia cooperation against the terror group as international powers try to restart stalled peace talks.

Russia and Syria argue US-backed rebel factions are close to al-Nusra and operate in the same battle space, thereby making them legitimate targets. Previous international talks to stop fighting broke down in part due to continued Russian and Syrian bombing of so-called moderate rebels with links to al-Nusra.

The offensive comes days after Syria and Russia said they had opened humanitarian corridors out to eastern Aleppo to let civilians flee and give rebel fighters the chance to surrender. Only a trickle of civilians have so far used the corridors.

The opposition has dismissed the Russian controlled corridors as nothing more than propaganda and psychological warfare.

The United Nations has voiced concern that the corridors could be used to depopulate the city to help government forces recapture areas under rebel control.

cw/cmk (AFP, Reuters)