The US supplies the materials to Syrian Democratic Forces (SDF) units to fight the Islamic State (IS) group, according to the Pentagon.

"We continue to provide very tailored arms and vehicles to the SDF" for use against IS, said Chris Maier, the director of the working group on the fight against the jihadists at the Department of Defense.

"We're very transparent about what those supplies are," Maier said.

"We provide monthly to Turkey a report of what those arms and vehicles are."

The US-led coalition against IS has relied on the SDF, an alliance of Arab and Kurdish fighters, to root out and prevent a resurgence of IS.

Turkey, with which the US has established a security buffer zone along the Syrian-Turkish border, considers the Kurdish portion of the SDF, called the People's Protection Units (YPG), to be a terrorist group.

Maier, who calls the buffer area a "security mechanism," said five joint helicopter flights by the US and Turkish militaries had already been conducted and the project's first joint ground patrol took place September 8.

Some Kurdish forces have left and been replaced by Arab fighters, though Maier said there are still YPG forces in the area.