A US-backed alliance of Syrian Kurds and Arabs will lead the assault on the Islamic State group's stronghold of Raqa, but rejects any Turkish role, its spokesman said Thursday.

"We will see a campaign led by the Syrian Democratic Forces to liberate Raqa city," SDF spokesman Talal Sello said.

"The topic of Turkey's participation was settled with the (US-led anti-IS) coalition definitively. No Turkish participation," he said.