US-backed Kurdish forces on Monday pleaded with Washington to halt a Turkish offensive against them as it prepared to send reinforcements to the region.

Turkey’s forces have crossed into northern Syrian to assault the Kurdish-held area of Afrin, raising alarm in Western capitals that Turkish troops are attacking the same fighters who helped defeat the Islamic of Iraq and the Levant (Isil).

Turkey says it is targeting the Kurdish People’s Protection Units (YPG), which it considers to be a terrorist group. But YPG fighters also make up the bulk of the Syrian Democratic Forces (SDF), the US-backed rebel group who drove Isil out of its capital in Raqqa.

As Turkish forces intensified their attack, the SDF pleaded with their US allies to restrain Turkey. “The coalition is urged to take its responsibilities towards our forces and our people in Afrin,” said Keno Gabriel, an SDF spokesman.

He warned that the Turkish attack on Kurdish troops would only benefit the remaining fragments of Isil and give them space to regroup. “This Turkish intervention comes to make final victory hollow,” he said.

The SDF said 18 civilians, including women and children, had been killed so far in the offensive. Turkey denied the claim, calling it “nonsense propaganda and baseless lies”.