No decision on who carries out an operation to capture Syria's Islamic State stronghold of Raqqa has yet been made, Turkish President Tayyip Erdogan's spokesman said on Thursday.

"Our plan is very clear, but the negotiation process is going on," Ibrahim Kalin said, referring to a trilateral meeting between U.S., Russia and Turkey military chiefs this week.

"There is no final decision on by whom and how the offensive on Raqqa will be carried out. Kurdish militants should move out of Syria's Manbij to the east of the Euphrates (river) as we were promised," Kalin told reporters in Ankara.