The opposition Syrian Observatory for Human Rights said the group was retreating east toward its stronghold in the city of Raqa.

The withdrawal comes four days after the Al-Qaeda affiliated Al-Nusra Front issued ISIL an ultimatum to go before a panel for mediation or face being forced from Syria altogether.

The deadline expires on Saturday.

"ISIL has withdrawn from Aazaz, its most important bastion in Aleppo province, as well as the Minnigh military airport, the Mayer region and the villages of Deir Jamal and Kafin," the Observatory said.

"Aleppo region is their weakest link, so they fear being attacked there" by Al-Nusra and other terrorist groups after the deadline expires, Observatory director Rami Abdel Rahman told AFP.

"ISIL is heading to areas that neighbor Raqa province where it has its main stronghold in the city of Raqa," he said.

ISIL fighters had regrouped in particular in Jarabulus and Manbij, on the far eastern border of Aleppo province and close to Raqa, he added.

Al-Nusra issued the threat against ISIL after the death of a senior rebel commander, Abu Khaled al-Suri, who had close ties to Al-Qaeda chief Ayman al-Zawahiri and his predecessor Osama bin Laden.

Rival terrorists accuse ISIS of killing the commander and Al-Nusra chief Abu Mohamed al-Jolani warned on Tuesday the group would be pushed out of Syria if it refused arbitration before the panel.

Both Al-Nusra and ISIL have roots in Al-Qaeda's onetime Iraqi affiliate, the Islamic State of Iraq.

But the two groups have never merged, with Jolani rejecting a union proposed by ISIL and Al-Qaeda's Zawahiri urging ISIL to return to Iraq after they moved into Syria.

Al-Nusra initially remained out of the fray but in recent weeks has begun actively fighting ISIL.

BA/BA