WASHINGTON (AFP) - President Donald Trump's White House vowed on Wednesday (April 4) that the US mission in Syria would come to a "rapid end" but failed to put a timetable on an eventual withdrawal.

Just days after Trump shocked aides and allies by declaring that the 2,000 troops would come home soon, US officials moved to scotch talk of a precipitous pullout.

The decision will reassure those in Washington pushing for a longer commitment to countering Russian and Iranian influence in a Syria still wracked by civil war.

But the hawks' relief may not last; the White House statement made it clear that the mission will continue only as long as it takes to defeat the beleaguered Islamic State in Iraq and Syria (ISIS) group.

And, with only a few thousand ISIS fighters thought to remain, the rationale for keeping US troops alongside Kurdish and Arab militia allies in the east of Syria may soon expire.

"The military mission to eradicate ISIS in Syria is coming to a rapid end, with ISIS being almost completely destroyed," the White House said.

"The United States and our partners remain committed to eliminating the small ISIS presence in Syria that our forces have not already eradicated," it added.

This commitment was lacking last week, when Trump triggered speculation of an imminent withdrawal by lamenting that America had wasted US$7 trillion (S$9.2 trillion) fighting in the Middle East.

"We'll be coming out of Syria, like, very soon. Let the other people take care of it now," he vowed, in a speech.

This appeared to contradict or undermine the policy outlined in January that troops would stabilise areas recaptured from the ISIS militants and boost US influence over Syria's future.

Then secretary of state Rex Tillerson argued US forces must remain to prevent ISIS and Al-Qaeda from returning and to deny Iran a chance "to further strengthen its position in Syria."

And he warned that "a total withdrawal of American personnel at this time would restore" Syrian strongman Bashar al-Assad to "continue his brutal treatment against his own people."

Trump has since sacked Tillerson, but US military commanders and diplomats have continued to work to implement the policy, including efforts to stabilise eastern Syria.

LASTING DEFEAT

As recently as Tuesday, the commander of US troops in the Middle East, General Joe Votel, said: "Of course there is a military role in this, certainly in the stabilisation phase."

And US diplomat Brett McGurk, the president's special envoy for the war against ISIS, said much work remains to be done to ensure the group's lasting defeat.

But questions remain over a US$200 million US pledge to pay for this endeavour, and Trump has publicly suggested that Gulf allies like Saudi Arabia should step up to pay.

The White House's decision appears to grant the mission a reprieve, but if Trump does not want to stay the course, he may leave room for Russia and Iran to step in.

As Trump was meeting his commanders in Washington, the presidents of Turkey, Russia and Iran were issuing a joint statement calling for a "lasting ceasefire" in Syria.

The three have been working to find a political solution in Syria under the Astana process, which began last year in competition with the US and UN-backed Geneva initiative.

Both Tehran and Moscow have deployed military forces to Syria to secure their own interests and to back up Assad in his now seven-year-old war against anti-government rebels.

Washington is a bitter foe of Iran's and has criticised Russia's support for a regime accused of slaughtering and starving civilians - and of using banned chemical weapons.

But US allies and Trump's more hawkish advisers fear a rapid American departure from eastern Syria would oblige America's Kurdish allies there to seek a compromise with Damascus.

And Iran would be free to extend its role to the Iraqi border, creating a continuous zone of influence between Teheran and its Lebanese ally Hezbollah, which has also deployed to Syria.

America's European allies also want the troops to stay on. A British SAS soldier was killed alongside a US comrade in Syria last week and France is reportedly sending reinforcements.

On Wednesday, the United States, Britain, France and Germany marked the first anniversary of an alleged Syrian sarin attack that killed 80 people by vowing to hold Assad to account.