Clashes between Turkish forces and units affiliated with a US-backed Kurdish-led alliance in Syria are "unacceptable", the Pentagon said Monday, calling on all sides to "stand down".

Clashes between Turkish forces and units affiliated with a US-backed Kurdish-led alliance in Syria are "unacceptable", the Pentagon said Monday, calling on all sides to "stand down".

In a statement sent to AFP, Pentagon spokesman Peter Cook condemned the fighting south of the Syrian town of Jarabulus.

"We are closely monitoring reports of clashes south of Jarabulus -- where ISIL is no longer located -- between the Turkish armed forces, some opposition groups, and units that are affiliated with the SDF (Kurdish-led Syrian Democratic Forces)," he said.

"We want to make clear that we find these clashes unacceptable and they are a source of deep concern."

The United States "was not involved in these activities, they were not coordinated with US forces, and we do not support them," he said.

"This is an already crowded battle space. Accordingly, we are calling on all armed actors to stand down immediately and take appropriate measures to de-conflict."

The comments come after Turkish forces began a two-pronged operation against ISIL and Kurdish fighters from the People's Protection Units (YPG) inside Syria on Wednesday.

The YPG is the main component of the US-backed SDF alliance, which has been fighting ISIL in northern Syria.

Turkey considers the YPG a "terrorist" group and said Monday it would continue to target the group if it failed to retreat east of the Euphrates River.

Turkish forces backed by pro-Ankara rebels seized the town of Jarabulus from ISIL on the first day of the operation, but have since then clashed with local fighters affiliated with the SDF.