It was another blow to the rebel movement. The momentum in the civil war has shifted in favor of Mr. Assad, whose forces have rolled back a number of rebel gains near Damascus, the capital, and elsewhere. Infighting among rebels who took up arms to topple Mr. Assad has allowed his forces to solidify their hold on central Syria and gradually expand their reach.

Skirmishes between groups are increasingly common, and many fighters have gotten bogged down in local turf wars.

Clashes have been raging for days across the ethnically mixed strip of land along the northern border with Turkey, pitting mainstream rebels and extremist groups linked to Al Qaeda against Kurdish militias. The Kurds, Syria’s largest ethnic minority, seek greater control of their own areas and fight to keep the rebels out.

On Sunday, Kurdish fighters surrounded the local leader of one of the groups linked to Al Qaeda, the Islamic State of Iraq and Syria, inside a school he and his fighters were using as a base near the border town of Tel Abyad. Activists said the Kurdish fighters did not storm the school to detain the group’s leader, who is known by the nom de guerre Abu Musab, because they feared he would blow up the school.

To push the Kurds to grant him safe passage, rebels and the group’s fighters detained hundreds of Kurdish civilians. The two sides finally reached a deal and the group’s leader was released in return for the 300 detained Kurdish civilians, according to the Syrian Observatory group.