Trump withdrawing troops because Syria is 'sand and death'

US President Donald Trump on Wednesday offered a rather stark take on the situation in war-wracked Syria, summing it up in two words - "sand and death" - while remaining vague about the timing of the withdrawal of US troops.

"So Syria was lost long ago. It was lost long ago. And besides that, I don't want - we're talking about sand and death. That's what we're talking about," Trump said during a cabinet meeting.

"We're not talking about vast wealth. We're talking about sand and death."

On when US forces would leave Syria, Trump said: "I don't want to be in Syria forever."

But after saying troops would come home right away, and that the Islamic State group was defeated, he didn't give any specific timetable.

"I never said we are getting out overnight," Trump said.

"Oh, we're withdrawing," he added, saying only that it would happen "over a period of time."

On Monday, the Republican president said the United States was "slowly sending" troops home - a markedly different tone than he used in his initial announcement of a withdrawal on December 19.

"Our boys, our young women, our men - they're all coming back and they're coming back now," he said in a video message that day.

The US has about 2,000 troops - most of them special forces - deployed in northern Syria to fight IS and reinforce anti-jihadist forces.

Top-ranking US military officials had repeatedly warned against a hasty retreat from Syria, which they say could leave the door open in Syria to allies of President Bashar al-Assad - notably Russia and Iran.

Trump's defence secretary Jim Mattis resigned over the decision to leave Syria, as did the administration's special envoy to the anti-IS coalition, Brett McGurk.