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.

Now, as some members of the force begin to peel away, officials fear the group may be on the verge of breaking up. For the U.S. military, their departure also means that fighters it supported and armed in the fight against Islamic State now are gearing up to battle Turkey, an ally and fellow member of the North Atlantic Treaty Organization.

Over the weekend, the Syrian Military Council, a largely Assyrian Christian force that fought Islamic State in several cities, announced on Twitter that it had “decided to send reinforcements to [the] Afrin battle to defend [the] Afrin people from Turkish invasion and attacks and its terrorist mercenaries.”

Kurdish fighters also have said they are planning to head to Afrin, based on social media messages verified by military officials.