France and Britain pledged to keep their forces in the field, warning that, contrary to Trump's claim that IS had been "beaten badly", the fight was far from over.

Around 2,000 US forces are in the country, most of them on a train-and-advise mission to support local forces fighting IS. London and Paris have not disclosed how many military personnel they have in the region.

""We remain committed to the global coalition and the campaign to deny Daesh territory and ensure its enduring defeat, working alongside our critical regional partners in Syria and beyond," a spokesman for Britain's Prime Minister Theresa May said, using an alternative name for IS.

"The global coalition against Daesh has made huge progress, but much remains to be done and we must not lose sight of the threat they pose," he added.

German Foreign Minister Heiko Maas and the Kurdish-dominated force leading the fight against the jihadists said the move could jeopardise the gains made against the Sunni radicals, who still control small parts of eastern Syria.

A withdrawal could also have wider geopolitical ramifications.

Some US lawmakers warned it could encourage Turkey to attack the Kurdish forces that the US has been supporting in the fight against IS.

- 'We've won' -

Trump on Wednesday abruptly ordered the withdrawal of all 2,000 US soldiers, mainly special forces, deployed in northeastern Syria, in a short video posted on Twitter.

"We've won against ISIS.... We've beaten them and we've beaten them badly. We've taken back the land. And now it's time for our troops to come back home," he said.

The announcement, which appeared to surprise both Republicans and Democrats in Congress, left unanswered a number of questions, notably as to the timing of the pullout and whether the US will continue to offer air support to its allies in the region.

Trump did not address the air campaign, in which the United States and partner nations including France and Britain have spent years pounding IS targets across the country.

Pentagon spokeswoman Commander Rebecca Rebarich said the US would continue its air war while American soldiers were in Syria -- but would not say if the campaign would continue afterward.

IS has seen its self-proclaimed 'caliphate', which once straddled both sides of the Iraq Syria border, shrink to just a few strongholds along the Euphrates Valley.

Experts warn that it still however has the power to conduct a guerrilla insurgency.

Britain and France said they remained committed to vanquishing the group which has directed or inspired several bloody attacks in the West.

"It's true that the coalition has made significant progress in Syria, but this fight continues, and we will continue it," French European Affairs Minister Nathalie Loiseau said.

- Russian satisfaction -

Analysts have said that a US pullout could give Russia and Iran -- which have been propping up the administration of Syrian President Bashar al-Assad -- a free hand in shaping the country's future.

Speaking in Moscow during his annual news conference, Russian President Vladimir Putin welcomed the US withdrawal, saying Trump was "right" to do so.

Meanwhile, Turkish President Recep Tayyip Erdogan and his Iranian counterpart Hassan Rouhani held talks in Ankara amid warnings that Turkey might launch a new military offensive in northern Syria against the Kurdish People's Protection Units (YPG).

Ankara considers the US-backed militia a "terrorist offshoot" of the Kurdish insurgents seeking autonomy for Kurds inside Turkey.

While Turkey has repeatedly called for the ouster of Bashar al-Assad, it has also sought to develop a pragmatic relationship with Tehran and Moscow who have helped turn the seven-year-old Syrian civil war in Assad's favour.

- 'Time for others to finally fight' -

Trump, who called for the withdrawal of US forces from the Middle East during his election campaign, defended his decision Thursday.

"Does the USA want to be the Policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing? Do we want to be there forever? Time for others to finally fight..," he declared in a Tweet.