US President Donald Trump said on Wednesday the United States would get out of Syria "over a period of time" and that he wants to protect the US-backed Kurdish fighters in the country as Washington draws down troops.

Trump did not provide a timetable for the planned military exit from Syria, which he described as "sand and death".

The US president unexpectedly announced the troop pull-out last month against the advice of top national security aides and without consulting politicians or US allies participating in the fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

Defense Secretary James Mattis unexpectedly resigned after the announcement and Brett McGurk, the US's top envoy in the fight against ISIL, announced he would be leaving his post earlier than expected due to the decision.

No timetable

During a Cabinet meeting at the White House in front of reporters, Trump said he had never set a reported four-month timetable for the withdrawal of 2,000 American troops stationed in Syria amid a battle against ISIL.

"We're getting out and we're getting outsmart," Trump said. "I never said I'm getting out tomorrow." He declined to say how long troops would remain in Syria.

In recent days, Trump appeared to back off from the rapid pull-out he initially ordered and stressed that the operation would be slow.

"We're slowly sending our troops back home to be with their families, while at the same time, fighting Isis [Islamic State] remnants," he said on Twitter on Monday.

Republican Senator Lindsey Graham said he came out of a recent lunch with Trump feeling reassured about the Syria policy.

Graham told reporters that Trump was committed to making sure Turkey did not clash with the Kurdish People's Protection Units (YPG) forces once US troops leave Syria and was assuring the NATO ally that it would have a buffer zone in the region to help protect its own interests.

Ankara views YPG as a terrorist organisation and claims the group is an extension of the Kurdistan Workers' Party (PKK), which has waged attacks on Turkish soil since the 1980s.

YPG weapons

US commanders planning the withdrawal are recommended that YPG fighters battling ISIL be allowed to keep US-supplied weapons, according to Washington officials.

That proposal would likely anger Turkey, where Trump's national security adviser, John Bolton, is expected to hold talks in the coming days.

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Trump said he was not happy that the Kurds were selling oil to Iran, but that he wanted to protect them either way.

"I didn't like the fact that they're selling the small (amount of) oil that they have to Iran, and we asked them not to sell it to Iran... We're not thrilled about that. OK? I'm not happy about it at all," he said.

"We want to protect the Kurds, nevertheless. We want to protect the Kurds, but I don't want to be in Syria forever. It's sand. And it's death."

Trump has been critical of previous administrations for sending and keeping US troops abroad, and has made bringing troops home part of his signature "America First" policy as he eyes a re-election bid in 2020.