AKCAKALE, Turkey — Turkish forces and their Syrian Arab allies swept into northern Syria on Thursday, killing at least 23 Kurdish fighters in a sharp escalation of a campaign against a Kurdish-led militia that had fought alongside the United States against the Islamic State.

The Turkish assault, which began on Wednesday after President Trump acceded to it and moved American troops out of the way, has pulled the militia out of its fight against the remnants of the Islamic State in Syria.

Three American officials said Thursday that all counterterrorism operations by the militia, the Syrian Democratic Forces, had been suspended.

The officials, speaking on condition of anonymity to discuss confidential military assessments, said the militia had been carrying out several dozen missions per day, some in conjunction with American Special Forces.