Groups that have received support from the United States or its allies have turned their guns on each other in a northern corner of Syria, highlighting the difficulties of mobilizing forces on the ground against ISIS.
As they fought among themselves before reaching a tenuous ceasefire on Thursday, ISIS meanwhile edged closer to the town of Azaz that was the focal point of the clashes near the border with Turkey.
Combatants on one side are part of a new U.S.-backed alliance that includes a powerful Kurdish militia, and to which Washington recently sent military aid to fight ISIS.
Their opponents in the flare-up include rebels who are widely seen as backed by Turkey and who have also received support in a U.S.-backed aid program.
Despite the ceasefire, reached after at least a week of fighting in which neither side appeared to have made big gains, trust remains low: each side blamed the other for the start of fighting and said it expected to be attacked again. A monitoring group reported there had still been some firing.
The fighting is likely to increase concern in Turkey about growing Kurdish sway near its border.
It also poses a new challenge for the U.S.-led coalition which, after more than a year of bombing ISIS in Syria, is trying to draw on Syrian groups to fight on the ground but finding many have little more in common than a mutual enemy.
Azaz controls access to the city of Aleppo from the nearby border with Turkey. It also lies in an area coveted by ISIS, which advanced to within 10 km (six miles) of the town on Tuesday and took another nearby village later in the week.
The fighting pitched factions of the Free Syrian Army, supported by Turkey and known collectively as the Levant Front, against the YPG and Jaysh al-Thuwwar - both part of the Democratic Forces of Syria alliance backed by Washington.
The Syrian Observatory for Human Rights, a Britain-based group that monitors the conflict in Syria, said Levant Front was supported in the fighting by the Ahrar al-Sham Islamist group and the al Qaeda-linked Nusra Front.
Observatory director Rami Abdulrahman said the rebels had received "new support, which is coming in continuously" from Turkey, a U.S. ally in the fight against ISIS.
"Turkish groups against U.S. groups -- it's odd," he said.
Although the YPG has been the most effective partner on the ground for theUnited States in the fight against ISIS, Turkey does not want tosee its influence expand further, even if the group is fighting ISIS.
The United States and Turkey have for months been talking of a joint effort to clear ISIS from the remaining part of the frontier, but there has been no sign of progress.
نستخدم ملفات الكوكيز لنسهل عليك استخدام مواقعنا الإلكترونية ونكيف المحتوى والإعلانات وفقا لمتطلباتك واحتياجاتك الخاصة، لتوفير ميزات وسائل التواصل الاجتماعية ولتحليل حركة المرور لدينا...اعرف أكثر