BEIRUT: Islamic State group jihadists have released hundreds of civilians used as human shields while fleeing a crumbling stronghold in northern Syria, but the fate of others remained unknown Saturday.

The last remaining IS fighters abandoned Manbij near the Turkish border on Friday after a rout that the Pentagon said showed the extremists were "on the ropes".

The retreat from the city, which IS captured in 2014, marked the jihadists' worst defeat yet at the hands of the Syrian Democratic Forces (SDF), an Arab-Kurdish alliance supported by US air strikes.

Fleeing jihadists took around 2,000 civilians, including women and children, on Friday to ward off air strikes as they headed to the IS-held frontier town of Jarabulus, according to the SDF.

At least some of the civilians were later released or escaped, the alliance said on Saturday, but the whereabouts of the rest was unknown.

"There are no more IS fighters" left in Manbij, an SDF member said.

Kurdish television showed footage of jubilant civilians in Manbij, including smiling mothers who had shed their veils and women embracing Kurdish fighters.

A woman burned a black robe that the jihadists had forced residents to wear, while men who had lived for weeks under a shaving ban cut their beards with scissors.