Turkey supports plans to drive Islamic State (IS) militants out of its Syrian stronghold of Raqqa but U.S.-backed Kurdish militia fighters should not be at the core of the operation, Turkish Defence Minister Fikri Isik said on Thursday.

Turkey launched its first major military incursion into Syria just over two weeks ago. Washington has said it supports the effort to push back IS, but the two NATO allies remain at odds over the role Kurdish fighters should play.

The United States sees the Syrian Kurdish YPG as a useful ally in the fight against the jihadists. But Turkey views them as a hostile force, an extension of Kurdish militants who have waged a three-decade insurgency on its own soil.

Speaking to Reuters after a meeting with U.S. Defense Secretary Ash Carter in London, Isik said YPG fighters should not be at the heart of any bid to seize Raqqa.

"What Turkey focuses and insists on is that instead of solely the YPG forces, the operations must be conducted, as the core of the operatives, by the local people of the region, instead of the YPG," he said in an interview.

"Turkey will not allow YPG forces to extend their territory and gain power by using the Daesh operations as an excuse," he said, using an acronym for IS.

Turkey and the United States are already at odds over control of another town, Manbij, some 30 km (20 miles) south of the Turkish border, which was captured last month from IS by a U.S.-backed coalition that includes the YPG.

Turkey has insisted that the Kurdish fighters pull back east of the Euphrates river following the operation, and warned that they will be legitimate targets for Turkish forces and the Syrian rebels Turkey backs if they fail to do so.

CONTROLLING BORDER WITH TURKEY

Carter told reporters that YPG fighters had already pulled out of Manbij and pledged to stay east of the Euphrates, but Isik said they had yet to fully do so.

Turkey fears that by advancing westwards from the Euphrates the Kurds would control unbroken tracts of borderland.

"We are expecting this pledge to be fulfilled. However the information we are receiving from our intelligence units and local sources indicate that YPG forces have not retreated to the east of the river. They are still present in the area," he said.

Turkey would seek no other objective around Manbij other than the retreat of the YPG forces, Isik said.

Carter acknowledged Turkish concerns but said U.S. policy in both Syria and Iraq was for local populations to rule cities and towns after IS's defeat.

"Nobody's going to go in there and occupy those cities except the people who already live there. That's the point," he said, adding that the YPG understood this and would not be allowed to hold Manbij.

Turkey's incursion into Syria came after it had called in vain for several years for world powers to help create a "safe zone" along the Syrian border, with the aim of clearing out IS and Kurdish fighters and of stemming a wave of migration that has caused tension with Europe.

Western allies have so far balked at the idea, saying it would require a significant ground force and planes to patrol a "no-fly zone", a major commitment in such a crowded and messy battlefield.

Turkish Foreign Minister Mevlut Cavusoglu told a joint news conference with his Saudi counterpart on Thursday that Turkish forces would nonetheless continue "north to south".

In London, Carter assured the Turkish defence minister of continued U.S. support for Turkey's efforts to clear IS from its borders. The two men also discussed the importance of local forces being at the centre of any Raqqa operation.