Islamic State group fighters on Friday seized around 2,000 civilians as they fled the former jihadist stronghold of Manbij in northern Syria, US-backed forces and a monitoring group said.

The Arab-Kurdish alliance known as the Syrian Democratic Forces (SDF) expelled most IS fighters from the town last week, but a small number remained.

"While withdrawing from a district of Manbij, jihadists abducted around 2,000 civilians," the SDF said.