Despite mutual enmity between these Kurdish groups and Damascus, where the Baathist government systematically persecuted Syrian Kurds, the SDF has seldom clashed with the Syrian government during the war.

The sides have also at times appeared to co-ordinate military campaigns against shared enemies, notably the Turkey-backed rebels taking part in the offensive with the Turkish army.

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The announcement came hours after US officials said American troops will leave northern Syria entirely.

Defence Secretary Mark Esper said Sunday, local time, that President Donald Trump had directed US troops in northern Syria to begin pulling out "as safely and quickly as possible." He did not say Trump ordered troops to leave Syria, but that seemed like the next step in a combat zone growing more unstable by the hour.

Esper said the administration was considering its options.

"We have American forces likely caught between two opposing advancing armies and it's a very untenable situation," Esper said.

This seemed likely to herald the end of a five-year effort to partner with Syrian Kurdish and Arab fighters to ensure a lasting defeat of the Islamic State group. Hundreds of IS supporters escaped a holding camp amid clashes between invading Turkish-led forces and Kurdish fighters, and analysts said an IS resurgence seemed more likely, just months after Trump declared the extremists defeated.

The US has had about 1000 troops in north-eastern Syria allied with the Kurdish-led Syrian Democratic Forces to combat IS. The Pentagon previously had pulled about 30 of these troops from the Turkish attack zone along the border. With an escalation of violence, a widening of the Turkish incursion and the prospect of a deepening conflict, all US forces along the border will now follow that move. It was unclear where they would go.

The Pentagon chief did not say US troops are leaving Syria entirely. The only other US presence in Syria is at Tanf garrison, near Syria's eastern border with Jordan. The US and coalition troops there are not involved in the Kurd mission, and so it seems highly unlikely the 1000 being moved from the north would go to Tanf.

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Critics say the US has betrayed the Kurds by pulling back in the face of Turkey's invasion, but Esper said the administration was left with little choice once President Recep Tayyip Erdogan told Trump a week ago that he was going ahead with a military offensive. Esper said the Kurds have been good partners, "but at the same time, we didn't sign up to fight the Turks on their behalf."

The prospect of enhancing the Syrian government's position on the battlefield and inviting Russia to get more directly involved is seen by Trump's critics as a major mistake. But he tweeted that it shouldn't matter.

"Others may want to come in and fight for one side or the other," he wrote. "Let them!"

New Jersey Senator Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, said Trump is weakening America. 'To be clear, this administration's chaotic and haphazard approach to policy by tweet is endangering the lives of US troops and civilians," Menendez said in a statement. "The only beneficiaries of this action are ISIS, Iran and Russia."

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The fast-moving developments were a further unravelling of US counterterrorism efforts in Syria, and they highlighted an extraordinary breakdown in relations between the United States and Turkey, NATO allies for decades. Turkish troops have often fought alongside American troops, including in the Korean War and in Afghanistan.

Asked whether he thought Turkey would deliberately attack American troops in Syria, Esper said, "I don't know whether they would or wouldn't."

He cited an incident on Friday in which a small number of US troops fell under artillery fire at an observation post in the north. Esper called that an example of "indiscriminate fire" coming close to Americans, adding it was unclear whether that was an accident.

Treasury Secretary Steven Mnuchin held out the possibility of quick action to impose economic sanctions on Turkey, a move that Trump has repeatedly threatened if the Turks were to push too far into Syria.

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"If we go to maximum pressure, which we have the right to do - at a moment's notice the president calls me up and tells me - we will do this," Mnuchin said. "We could shut down all US dollar transactions with the entire government of Turkey. ... That is something we may do, absolutely."

European Union leaders are meanwhile seeking to broaden an arms embargo against NATO ally Turkey after Germany, France and the Netherlands suspended their sales to Turkey.

Speaking at the EU foreign ministers meeting in Luxembourg, Dutch Foreign Minister Stef Blok said on Monday "it is against international law to invade a neighboring country and Turkey should as every other country abide by international law".

AP