"Such steps are extremely dangerous for Syria's unity and territorial integrity," Foreign Minister Mevlut Cavusoglu said, after the United States began providing small arms to the Kurdish fighters.
"If we are looking for stability in Syria, we should row back from those mistakes," he told a press conference with Slovenian counterpart Karl Erjavec.
The Pentagon on Tuesday said it had begun to transfer small arms and vehicles to the Kurdish elements of the Syrian Democratic Forces, a Kurdish-Syrian Arab alliance fighting the Islamic State group and containing Kurdish Peoples' Protection Units (YPG) fighters.
Related reading
Turkey kills 13 Kurdish militants in northern Iraq
The weapons include AK-47s and small-caliber machine guns, Pentagon spokesman Major Adrian Rankine-Galloway said.
Turkey views the YPG as a "terror group" linked to Kurdish separatists waging an insurgency inside Turkey since 1984 that has killed more than 40,000 people.
But Washington believes the YPG is the most effective fighting force against IS jihadists in Syria, thus causing tensions between the NATO allies.
The US' weapons transfers began ahead of an upcoming offensive to recapture Raqa, the last major bastion for IS in Syria.
The SDF have now advanced to within a few miles of Raqa on several fronts, and this month captured the strategic town of Tabqa and the adjacent dam from the jihadists.
Turkish President Recep Tayyip Erdogan met with US counterpart Donald Trump in Washington for the first time where the issue of US support for the YPG was discussed on May 16, the Turkish foreign minister said.
Less than a week before Erdogan's visit, Trump approved arming fighters from the YPG.
"The president clearly expressed our position and concerns during his Washington visit. It was stressed how risky and dangerous the support given to the YPG was," Cavusoglu said.
"These weapons could be used against all humanity, not just Turkey."
