WASHINGTON — The United States-led campaign to hunt down the last pockets of Islamic State militants in Syria has lost its most effective battleground partner in what American military officials fear will stall a critical phase of the offensive and leave open the door for hundreds of foreign fighters to escape.

Thousands of Kurdish fighters and commanders who make up the backbone of the Syrian Democratic Forces in recent weeks have diverted to defend Afrin, in Syria’s northwest, where other Kurdish militia are facing sharp attacks from Turkish troops.

The Kurdish-led S.D.F. was the driving force last fall in routing the Islamic State from its self-proclaimed headquarters in Raqqa and chasing insurgents fleeing south along the Euphrates River Valley to the Iraqi border.

That fight now is largely reliant on Syrian Arab fighters who make up a majority of the S.D.F. but lack the Kurds’ military organization and logistical prowess.