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.

Raqa will be the last major city under the militants' control if forces in neighbouring Iraq win the battle now under way for Mosul.

Backed by US-led coalition air strikes, the SDF has flushed IS group out of swathes of territory in northern Syria, most recently the flashpoint town of Manbij.

It is dominated by the powerful Kurdish People's Protection Units (YPG).

The United States and Britain said in late October they expected an assault on Raqa -- IS's de facto capital in Syria -- within weeks.

"We are ready. We have the sufficient numbers for this campaign and we will start it soon," Sello told AFP, without specifying a timetable.

But he insisted that the fight would not include Turkey, which views the YPG as linked to the outlawed Kurdistan Workers' Party (PKK), considered by Ankara as a "terrorist" group.

"The topic of Turkey's participation was settled with the (US-led anti-IS group) coalition definitively.

No Turkish participation," he said.