Turkey has summoned the U.S. ambassador after a State Department spokesman said Washington did not regard Syria's Kurdish Democratic Union Party (PYD) to be a terrorist organization.
Ankara sees the PYD as a branch of the Kurdistan Workers Party (PKK), which is fighting for Kurdish autonomy in southeastern Turkey and is categorized as a terrorist organization by Turkey, the United States, and the European Union.
Washington supports the PYD in its struggle against Islamic State militants in Syria.
Turkish officials said U.S. Ambassador John Bass was called to the Foreign Ministry on February 9 where a senior Turkish official expressed Ankara's concerns.
It came after State Department Spokesman John Kirby said on February 8 that "we don't, as you know, recognize the PYD as a terrorist organization."
The issue has strained ties between the two allies and President Recep Tayyip Erdogan even called on Washington to choose between Turkey and the PYD as its partner.