A U.S.-backed operation by Syrian forces to capture Raqqa from Islamic State (IS) group will start in the coming "few days" after advances to the outskirts of the city, the spokesman for the Syrian Kurdish YPG militia said on Saturday.

The offensive is being waged by the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias backed by the U.S.-led coalition against IS group, also known as Daesh.

"The forces reached the outskirts of the city, and the major operation will start ... in the coming few days," YPG spokesman Nouri Mahmoud told Reuters by phone.

Mahmoud was confirming a report citing the spokeswoman for the Raqqa campaign, Jihan Sheikh Ahmed, as indicating a new phase to storm Raqqa would start in the "coming few days". The remarks made in an interview with a local media outlet were circulated by an SDF-run Whatsapp group.

A spokesman for the U.S.-led coalition against Daesh said it would not comment on the timeline for the next phase of operations to retake the eastern Syrian city from Daesh.

The spokesman, Colonel Ryan Dillon, said the SDF were "advancing closer and closer every day", having moved to within 3 km of Raqqa to the north and to the east.

To the west, the SDF were less than 10 km away, he said in emailed answers to questions from Reuters.