The jihadist Islamic State of Iraq and the Levant has begun withdrawing from parts of northern Syria ahead of a deadline set by a rival group, a monitor said Friday.
The Syrian Observatory for Human Rights said the group was retreating east towards 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 an Islamic court for mediation or face being forced from Syria altogether.
The deadline expires on Saturday.
"ISIS has withdrawn from Aazaz, its most important bastion in Aleppo province, as well as the Minnigh military airport, the Mayer region and the villages of Deir Jamal and Kafin,"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 rebels after the deadline expires, Observatory director Rami Abdel Rahman told AFP.
"ISIL is heading to areas that neighbour 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.
In the wake of the withdrawal from Aazaz, which ISIL seized in September 2013, the Observatory said a possible mass grave was found in the city.
The withdrawal was confirmed by the opposition Aazaz Media Centre, which claimed it as a victory for rival rebel fighters.
"God is great. The heros of the Free Syrian Army and the Northern Storm (brigade) have liberated the town of Aazaz from the dogs of Baghdadi," the centre wrote on its Facebook page, referring to ISIL leader Abu Bakr al-Baghdadi.
Video footage from the town posted online showed a demonstration of residents chanting "the Free Syrian Army forever" after ISIL's withdrawal.
- 'Jewel in ISIL crown' -
Charles Lister, a visiting fellow at the Brookings Doha Center, said ISIL appeared to have withdrawn from several locations without a fight.
"It looks like ISIL has made the strategic decision to reinforce existing strongholds in eastern Aleppo, all of which lie on valuable routes towards the jewel in ISIL's crown, the city of Raqa," said Lister.
"Removing these weak areas and reinforcing important and stronger ones seems like the only logical strategy left for ISIL at this stage," he added.
Al-Nusra on Tuesday issued a threat against ISIL after the death of a senior Islamist commander, Abu Khaled al-Suri, who had close ties to Al-Qaeda chief Ayman al-Zawahiri and his predecessor Osama bin Laden.
Rival rebels accuse ISIL of killing the commander and Al-Nusra chief Abu Mohamed al-Jolani warned on the group would be pushed out of Syria if it refused arbitration before an Islamic court.
Lister said a major offensive against ISIL could seriously affect the opposition's ability to hold territory against the regime, and that casualties in such an offensive would be high.
"As such, a compromise or a series of localised compromises could still be possible, but this would depend on ISIL playing diplomacy, which isn't necessarily a proven strength."
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 its fighters moved into Syria.
In early January, a coalition of Islamist and moderate rebels angered by ISIL's abuses of civilians and rival opposition fighters began attacking the group.
Al-Nusra initially remained out of the fray but in recent weeks has begun actively fighting ISIL.