Syrian rebel fighters loyal to al-Qaeda ceded ground near the Turkish border to rival rebel groups, activists have said, in what seemed to be a tactical withdrawal to end clashes between those opposed to the regime of President Bashar al-Assad.
Opposition activists said the Islamic State of Iraq and the Levant (ISIL), allied to al-Qaeda and featuring foreigners among its commanders, had pulled back on Sunday from strongpoints including al-Dana and Atma in Idlib province and that fighters from Jabhat al-Nusra and Ahrar al-Sham moved in.
Meanwhile, other fighters opposed to the rule of Assad seized a compound garrisoned by ISIL in Aleppo province, in some of the most serious infighting to date within the vast array of rebel groups trying to topple the government, activists said.
The clashes between a loose alliance of opposition brigades and the ISIL, which has sought to impose a strict interpretation of Islam on opposition-held areas, have spread across northern Syria in the last few days.
ISIL, which has many foreign fighters in its ranks, has clashed repeatedly with more moderate rebel groups since it aggressively pushed into Syria from neighbouring Iraq last spring.
The infighting has left scores dead on both sides, and has undermined the overall rebel movement's efforts to oust Assad.
The latest clashes broke out on Friday after residents accused ISIL members of killing a doctor in Syria's northern province of Aleppo.
The Islamic Front, a newly-created umbrella group of powerful, mostly conservative fighters, issued a statement ordering ISIL to hand over the doctor's killers so they can stand trial.
The group did not, sparking clashes between the factions in Aleppo province. Fighting quickly spread to rebel-held areas of the province of Idlib and the central province of Hama.
On Sunday, the violence widened again, with clashes in the town of Tabaqa in Raqqa province, said Rami Abdel Rahman of the Britain-based Syrian Observatory for Human Rights
The Observatory said forces loyal to ISIL had killed at least 50 rival rebels in northern Syria.
Some of the heaviest fighting took place in the town of Manbij in Aleppo province, where rebels seized an ISIL compound, activists said.
The Observatory said ISIL fighters used car bombs, a tactic usually reserved for attacking government forces, for the first time to defend its territory.
Popular resentment of ISIL has been brewing in northern Syria for months.
The group has employed tactics deemed brutal even by the standards of Syria's bloody conflict. Its fighters have beheaded captured government fighters, and kidnapped anti-Assad activists, journalists and civilians seen as critical of its rule.
As the civil war gets ever more complex amid a broad regional confrontation between Sunni and Shia Muslims, the United States raised the prospect of Iran playing some role in this month's Syrian peace talks in Geneva.