A powerful al-Qaeda affiliate in Syria has issued an ultimatum to other rebel factions that have been fighting it.
The Islamic State in Iraq and the Levant (Isis) threatened to leave the city of Aleppo to government forces unless its rivals stop their attacks against it within 24 hours.
Isis has suffered losses in two days of fighting against an alliance of rebel forces in Aleppo and Idlib provinces.
The group is accused of imposing a reign of terror on areas it controls.
In an audio message, an unidentified Isis spokesman called on other rebel groups to lift checkpoints and release all the prisoners they hold.
He said Isis had been "stabbed in the back", and that if the attacks did not stop, it would have no choice but to pull back from the frontlines of Aleppo.
Dozens of its members are reported to have been killed or captured over the past two days.
The BBC's Rami Ruhayem in neighbouring Lebanon says the fighting in northern Syria has highlighted a deep rift between groups that share a lot in ideology and aspirations.
Isis is fighting Free Syrian Army groups as well as the Islamic Front, a coalition of Syrian rebel factions which also wants to build an Islamic state in Syria.
The fighting flared on Friday, prompted partly - reports said - by Isis gunmen who fired at residents of the Aleppo village of Kafr Takharim. They had been protesting against the death of a doctor and rebel commander in Isis custody.
Other rebel groups say Isis has attempted to hijack their struggle for its own ends.
An Idlib activist, Abu Leyla, told AFP news agency that Isis "only benefits the Assad regime".
That was echoed by the main opposition National Coalition of Syrian Revolution and Opposition Forces, which applauded the FSA's efforts to dislodge Isis.
"Isis is an extension of the Assad regime," the Coalition's Munzer Akbik said.
"The solution to fighting extremism in Syria is to strengthen the Free Syrian Army at this critical juncture."
Fighting has been reported in:
- Atareb, Aleppo province, with FSA units reportedly making progress but Isis surrounding the town and shelling it
- The eastern districts of Kallassah, Ansari and Jisr al-Hajj in Aleppo city, where the Mujahideen Army is taking on Isis
- Qabtan al-Jabal and surrounding villages west of Aleppo city, with reports of dozens of Isis fighters captured
- Hazzano and Maarat Misreen in Idlib province
- But in Saraqeb and Kafranbel, Idlib, Isis is reported to be rounding up "suspect activists"
The fight against Isis appears to be the priority for all groups fighting the Syrian government, with one exception - the Nusra Front, our correspondent says.
Organically linked to Isis, the two groups used to follow the same al-Qaeda leadership until they split last year. Nusra is now closely allied to the Islamic Front, and has been trying to remain neutral.
For all the groups involved, the fighting is diverting precious resources away from the fight with the Syrian government, our correspondent adds.