Turkey has summoned the U.S. ambassador to "convey unease" a day after the U.S. State Department spokesman said that, unlike Turkey, Washington doesn't recognize Syrian Kurdish forces as "terrorists."

The issue has strained ties between the two allies and President Recep Tayyip Erdogan even called on Washington to choose between Turkey and the Kurdish Democratic Union Party, or PYD, as its partner.

A Foreign Ministry official said Ambassador John Bass was called to the ministry on Tuesday where a senior Turkish official expressed Ankara's concerns. The official spoke on condition of anonymity because she wasn't authorized to make public statements.

U.S. State Department spokesman John Kirby said Monday: "We don't, as you know, recognize the PYD as a terrorist organization. We recognize that the Turks do."