Radical jihadists begin withdrawing from parts of northern Syria Friday after a threat from rivals, in a bid to protect their stronghold in the east of the war-ravaged country.
The main bloc in Syria's opposition in exile, meanwhile, announced it would rejoin the umbrella coalition it split from in a bid to close ranks against the regime.
Al-Qaeda-affiliated Al-Nusra Front has threatened the Islamic State of Iraq and the Levant with all-out war if it does not submit by Saturday to mediation by an Islamic court over allegations ISIL assassinated an Islamist commander close to Al-Qaeda chief Ayman al-Zawahiri.
Since January, ISIL has been battling a coalition of moderate and Islamist rebels angered by its abuses of rival fighters and civilians, but Al-Nusra had largely stayed out of the fray.
The prospect of the powerful Al-Nusra joining with ISIL's opponents appears to have prompted the group to withdraw to its stronghold in the eastern city of Raqa, the only provincial capital lost by the regime in the three-year civil war.
The Syrian Observatory for Human Rights said ISIL had pulled back from positions in northern Aleppo province, including the key town of Aazaz.
"ISIL has withdrawn from Aazaz, its most important bastion in Aleppo province, as well as the Minnigh military airport, the Mayer region and the villages of Deir Jamal and Kafin,"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 Britain-based Observatory said.
-- Aleppo ISIL's 'weakest link' --
"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 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, 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 greatest. 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.
Footage posted online showed a demonstration of local 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.
"Removing these weak areas and reinforcing important and stronger ones seems like the only logical strategy left for ISIL at this stage," he said.
On Tuesday, Al-Nusra 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 Zawahiri and his predecessor Osama bin Laden.
Rebels accuse ISIL of killing the commander, and Al-Nusra chief Abu Mohamed al-Jolani warned 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.
Both Al-Nusra and ISIL have roots in Al-Qaeda's onetime Iraqi affiliate, the Islamic State of Iraq.
But the two 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.
As tensions between Islamist rebel groups rose, the mainstream opposition's Syrian National Council said it will rejoin the coalition from which it withdrew in protest at the Geneva peace talks with the regime.
The SNC, the biggest bloc in Syria's opposition-in-exile, quit the National Coalition on January 20 because of its participation in the talks.
In fighting on Friday, state news agency SANA said the army had killed 20 rebels in an ambush near Damascus, two days after state media reported the deaths of 175 rebels in a similar ambush nearby.