Turkey will not cooperate with the main Kurdish militia in Syria to attack the Islamic State's de-facto capital, al-Raqqa, President Recep Tayyip Erdogan's spokesman said Thursday.
"In principle, we support clearing Raqqa and other Syrian cities of Daesh [Islamic State]. However, our principles and conditions on this issue are clear," Ibrahim Kalin said, according to state-run news agency Anadolu.
Cooperating with the US-backed Kurdish militia, the People's Protection Units (YPG), was "out of the question."
The YPG and its Arab allies have siezed large swaths of territory in northern Syria from Islamic State, fighting in a formation known as the Syrian Democratic Forces, which is backed by US special forces and airstrikes.
Turkey has attacked the YPG on a number of occasions. Last month, Ankara, a NATO member, launched an invasion of northern Syria to hold off the YPG advancing and also to attack Islamic State, seizing villages along the border.
The operation has raised tensions with the Washington. US officials have long complained that Turkey has been reluctant in its fight against Islamic State, including a sluggish effort to shut its porous border.
The New York Times this week reported that Washington is considering arming the YPG directly, in a move that would anger Ankara.
Turkey is worried YPG gains could stoke Kurdish nationalism at home as its security forces battle a domestic Kurdish insurgency led by the Kurdistan Workers Party (PKK).
Kurds are a minority group in Turkey and Syria and complain about systematic discrimination in both countries.