Washington's top diplomat said he was "optimistic" an agreement with Ankara could be reached which will protect Syrian Kurdish groups while allowing Ankara to "defend their country from terrorists" following a United States pullout from Syria.

"We are confident we can achieve an outcome that achieves both of those," United States Secretary of State Mike Pompeo told journalists on Saturday in Abu Dhabi.

The Gulf emirate is his latest stop in a regional tour aimed at reassuring allies after a shock December announcement by President Donald Trump that US troops would be withdrawn from Syria.

Pompeo's remarks follow tensions between the US and Turkey over the fate of Washington's Syrian Kurdish allies in the fight against Islamic State of Iraq and the Levant (ISIL, also known as ISIS) fighters.

Turkey had reacted angrily to suggestions that Trump's plan to withdraw troops was conditional on the safety of the US-backed Kurdish fighters, seen by the Turkish government as terrorists.

US-led operations against ISIL in Syria have been spearheaded on the ground by the Kurdish-dominated Syrian Democratic Forces (SDF).

Ankara sees the backbone of that alliance, the Kurdish People's Protection Units (YPG), as a terrorist group linked to the Kurdistan Worker's Party (PKK) which has fought a decades-long campaign against the Turkish state.

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Pompeo said that Washington recognised "the Turkish people's right and (Turkish President Recep Tayyip) Erdogan's right to defend their country from terrorists".

But, he added, "We also know that those fighting alongside of us for all this time deserve to be protected as well."

Pompeo said he had spoken to Turkey's Foreign Minister Mevlut Cavusoglu, but did not elaborate on how the protections would be achieved.

"Many details (are) still to be worked out but I'm optimistic that we can achieve a good outcome," he said.

Threatened assault

Multiple operations including American-backed assaults have removed ISIL fighters from most of the swaths of Syria and Iraq they captured in 2014.

But Trump's announcement raised fears of a long-threatened Turkish assault against the Kurds.

On Thursday, Cavusoglu repeated that threat, telling NTV television: "If the (pullout) is put off with ridiculous excuses like Turks are massacring Kurds, which do not reflect the reality, we will implement this decision."

That came after a tense meeting between Turkish officials and Trump's national security adviser John Bolton in Ankara, aimed at coordinating the pullout process after Bolton set conditions that appeared to postpone it indefinitely.

The terms included the total defeat of ISIL - still active in some parts of Syria - and ensuring protection for Kurdish fighters.

The US-led coalition launched operations against ISIL in September 2014, forming the SDF a year later with some 25,000 Kurdish fighters and 5,000 Arabs.

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Backed by US arms and air support, the YPG-dominated group has overrun the de facto Syrian capital of ISIL, Raqqa, and a large part of Deir Az Zor province.

But that stirred Turkish fears of a breakaway Kurdish state on its border.

As well as fighting ISIL, the YPG has also battled pro-Ankara forces in northwestern Syria.

Trump's announcement last month prompted the YPG to call on Syrian government troops to deploy alongside their own forces in the north to help counter a potential Turkish offensive.

A spokesman for the US military said Friday it had begun "the process of our deliberate withdrawal from Syria".

But US defence officials quickly sought to clarify that while gear was being pulled out, "we are not withdrawing troops at this stage".

In an interview with US broadcaster CBS on Saturday, Pompeo said "the president's guidance is incredibly clear" on the withdrawal.

Syria's devastating conflict began in 2011 with anti-government demonstrations that were brutally crushed, sparking a complex war involving multiple foreign armed groups, as well as regional and international powers including the US.

The withdrawal announcement had also sparked concerns among Arab states and Israel that it could open the way to growing Iranian influence.

Pompeo has pledged to "expel every last Iranian boot" from Syria, and on Saturday sought to downplay the effect of the US pullout on this goal.

"The fact that a couple of thousands of uniformed personnel in Syria will be withdrawing is a tactical change," he said.

"It doesn't materially alter our capacity to continue to perform the military actions that we need to perform."

The US is looking to create an anti-Iran front - the Middle East Strategic Alliance - bringing together Gulf countries as well as Egypt and Jordan.

Washington is set to convene an international summit in Poland next month focusing on stability in the Middle East, including countering Iran's influence.