"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.

In a statement late Wednesday, Turkey's top National Security Council (MGK) said US support for the Kurdish Peoples' Protection Units (YPG) ran counter to the spirit of alliance.

"The policy of backing PKK/PYD-YPG terror organisation does not bode well with friendship and alliance," the MGK statement read without naming the United Staes, following a meeting chaired by President Recep Tayyip Erdogan.

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 YPG fighters.

The weapons include AK-47s and small-calibre machine guns, Pentagon spokesman Major Adrian Rankine-Galloway said.

Turkey views the YPG as a "terror group" linked to Kurdistan Workers' Party (PKK) separatists, who have waged an insurgency since 1984 that has killed more than 40,000 people inside Turkey

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 IS bastion 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 an adjacent dam from the jihadists.

Erdogan met with US counterpart Donald Trump in Washington for the first time on May 16, where the issue of US support for the YPG was discussed, the Turkish foreign minister said.

Less than a week before Erdogan's visit, Trump approved arming YPG fighters.

"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."