Turkish-backed rebels fought each other on Sunday in the Syrian town of al Bab in the first bout of rebel fighting since they seized the town this year from Islamic State militants, rebels and witnesses said.
It was not clear what triggered the armed clashes between rival rebel groups who hold sway in the city since it was seized last February from Islamic State militants.
Turkey sent warplanes, tanks and artillery into Syria last year in support of the Arab Sunni rebels, an operation dubbed "Euphrates Shield" which was aimed at driving both Daesh and Kurdish militia forces away from its border.
Checkpoints were spread by the Turkish-controlled military council that runs the city where thousands of people have been returning in recent months with the gradual resumption of basic services, a resident who was contacted in the city said.
The heavy clashes left several dead and scores injured, another rebel source said.
Rebel infighting has been a major weakness of the revolt against President Bashar al Assad's rule since its earliest days, with rebel factions divided by both ideology and local power struggles.
In the last few days deadly clashes have also broken out in the town of Maarat al Numan in the northwestern province of Idlib between Tahrir al Sham, a merger of jihadist groups spearheaded by a former al-Qaeda offshoot, and a Western-backed Free Syrian Army group.