An operation to retake the Syrian city of Raqqa from the so-called Islamic State has begun.

The US-backed Syrian Democratic Forces (SDF) alliance of Kurdish and Arab armed groups said it had set up a command centre to coordinate with the US-led coalition in the campaign to drive IS from its de facto Syrian capital.

In a press conference in the Syrian town of Ain Issa, a town about 60km northwest of Raqqa, the group also urged residents of the city to leave and avoid areas where the militants were present to avoid harm.

The Syrian YPG Kurdish force is one of the most powerful militias in Syria and regarded as the backbone of SDF. 

The operation comes as Iraqi forces backed by a US-led coalition press an assault to take the jihadist group's Iraqi stronghold of Mosul.