Members of a U.S.-backed force of Syrian fighters who played a critical role in the collapse of Islamic State have begun leaving that operation to counter Turkey’s offensive against Kurds along its border with Syria, according American officials and Syrian fighters.U.S. officials have voiced repeated concerns about Ankara’s assault in recent weeks against Kurds in the northwestern Syrian city of Afrin, warning it could become a security distraction that would draw the U.S.-backed force away from the fight to defeat remaining pockets of Islamic State militants.