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

"We’ve called on both sides to not fight with one another, to continue to focus the fight on ISIL," Defense Secretary Ashton Carter said on Monday, using an acronym for the Islamic State group (IS).

"That’s the basis of our cooperation with both of them -- specifically not to engage."

Turkish forces last week launched a two-pronged operation against IS and Kurdish fighters from the People’s Protection Units (YPG) inside Syria.

The YPG is the main component of the Kurdish-led Syrian Democratic Forces (SDF), which has been fighting IS in northern Syria.

The US-led coalition has been backing the YPG with training and equipment to fight IS, while at the same time the United States has also supported Syrian opposition groups fighting with the Turks in northern Syria.

Ankara has said it killed 25 Kurdish "terrorists" in strikes on YPG positions on Sunday -- meaning the two US-backed partner forces were fighting each other.

Pentagon press secretary Peter Cook condemned the fighting south of the Syrian town of Jarabulus, where Turkish armed troops had fought with the SDF.

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

Crossing the Euphrates

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

"The YPG elements of (the SDF) will withdraw, and is withdrawing, east of the Euphrates," Carter said.

"That will naturally separate them from Turkish forces that are heading down in the Jarabulus area."

Turkish forces backed by pro-Ankara rebels seized the town of Jarabulus from IS last week, but also clashed with local fighters affiliated with the SDF.

Two US defense officials said on Monday that the SDF withdrawal was essentially complete, but they acknowledged some Kurdish people remained to the west of the Euphrates.

Another defense official said some SDF forces remained in the city of Manbij, south of Jarabulus, which was this month wrested from IS.

US Vice President Joe Biden made a visit to Turkey last week, during which he said Washington was looking to preserve the territorial integrity of Syria.

And President Barack Obama will meet his Turkish counterpart Recep Tayyip Erdogan next weekend in China, on the sidelines of the Group of 20 summit, with Syria high on the agenda.

Dozens of people were killed in Turkish bombardments in Syria on Sunday as Ankara ramped up its unprecedented offensive.

Ankara said it had killed 25 Kurdish "terrorists" and insisted the army was doing everything possible to avoid civilian casualties.

But the Britain-based Syrian Observatory for Human Rights said at least 40 civilians were killed in Turkish shelling and air strikes on two areas held by pro-Kurdish forces, the first report of significant civilian casualties in Turkey’s operation. – AFP