More than 3,000 civilians have fled the Islamic State group's former Syrian stronghold Raqa, a spokesman for the US-backed force on the verge of capturing the city told AFP Sunday.

"More than 3,000 civilians left on Saturday night as part of a deal and went to areas controlled by the Syrian Democratic Forces," said Talal Sello, spokesman for the militia.

He said Raqa was now "completely free of civilians," with the exception of the families of IS fighters still in the city.