The U.S.-backed Syrian Democratic Forces (SDF) alliance of Kurdish and Arab armed groups said on Thursday it would reject Turkish involvement in the operation to drive Islamic State from its Syrian capital Raqqa.

U.S. officials have said they hope to start an offensive against Islamic State in Raqqa within weeks, and have said that the SDF will play a big role, but Washington's ally Turkey has also insisted that it take part in the operation.

"The Syrian Democratic Forces are the only force that will take part in the operation to liberate Raqqa and we informed the (international U.S.-led) coalition forces that we reject any Turkish role in the Raqqa liberation operation," SDF spokesman Talal Silo told Reuters.

Turkey's military and allied Syrian rebel groups last week fought against Kurdish forces allied to the SDF in northwest Syria, where both sides are seeking to take territory from their mutual enemy Islamic State.

Ankara is dismayed at the prominent role in the SDF played by the Kurdish YPG militia, which it sees as a front for the Kurdistan Workers Party that has waged a three-decade insurgency inside Turkey.

Since it was formed in early 2015, the SDF has seized large swathes of territory along the Syria-Turkey border from Islamic State and pushed the jihadist group back to within 30km (18 miles) of Raqqa.

An offensive by the Iraqi army and Kurdish forces, backed by the U.S., started last month to reclaim Mosul, the largest city under the control of the jihadist group, and one whose capture would leave Raqqa as Islamic State's most important possession.