The decision runs counter to long-established US policy for Syria and the region. It blindsided lawmakers, the Pentagon and international allies alike.

Britain and France warned on Thursday that the fight against jihadists in Syria was not finished.

Trump earlier said: "We've won against ISIS," in a short video posted on Twitter.

"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."

A withdrawal could have major geopolitical ramifications, and plunges into uncertainty the fate of US-backed Kurdish fighters who have been tackling Islamic State jihadists, thousands of whom are thought to remain in Syria.

A US official told AFP that Trump's decision was finalized Tuesday.

"Full withdrawal, all means all," the official said when asked if the troops would be pulled from across Syria.

Currently, about 2,000 US forces are in the country, most of them on a train-and-advise mission to support local forces fighting IS.

Pentagon officials scrambled for a reaction. A spokeswoman eventually said the Defense Department had "started the process" of bringing troops home.

Lawmakers assailed Trump's decision, saying it could embolden Ankara to attack US-backed Kurdish fighters.

Republican Senator Lindsey Graham, a Trump ally, said the president's decision was unwise and put the Kurds "at risk."

Democratic Senator Jack Reed said it amounted to a "betrayal" of the Kurds that "provides further evidence of President Trump's inability to lead on the world stage."

Blasting the move as a "huge Obama-like mistake," Graham said "I fear it will lead to devastating consequences for our nation, the region and throughout the world."

Most US troops are stationed in northern Syria, though a small contingent is based at a garrison in Al-Tanaf, near the Jordanian and Iraqi borders.

Trump has previously voiced skepticism about the US presence in Syria, saying in March he wanted to bring troops home "soon."

But military advisors and international allies warned Trump against a precipitous pullout, and he later acquiesced to an indefinite Syria mission.

The US official would not provide a withdrawal timeline, saying only it would come "as quickly as possible."

White House spokeswoman Sarah Sanders said the US-led coalition that includes dozens of nations would continue fighting the jihadists.

"These victories over ISIS in Syria do not signal the end of the Global Coalition or its campaign," Sanders said in a statement.

The Pentagon refused to say what effect the troop withdrawal would have on air operations in Syria that have been ongoing since late 2014.

A senior administration official said Trump's decision was consistent with comments he has made for years.

"The notion that anyone within the administration was caught unaware, I would challenge that," the official said.

- Fate of Kurdish fighters? -

A large contingent of the main US-backed, anti-IS fighting force in Syria, an alliance known as the Syrian Democratic forces (SDF), is Kurdish. Turkey terms it a "terrorist" group.

Ankara has said it plans to launch an operation against the Kurdish militia known as the YPG (Kurdish People's Protection Units).

While the YPG has spearheaded Washington's fight against IS, US support has strained relations between the NATO allies.

The US decision to withdraw from Syria marks a remarkable development not just for the Kurds, but for years-old US doctrine in the region.

Only last week, Brett McGurk, the special envoy to defeat IS, said "nobody is declaring a mission accomplished."

"If we've learned one thing over the years, enduring defeat of a group like (IS) means you can't just defeat their physical space and then leave," he said.

- 'Short-sighted and naive' -

A statement issued by the British government, which has long supported the anti-IS campaign in Syria, said "much remains to be done" against the jihadists.

"We must not lose sight of the threat they pose. Even without territory, (IS) will remain a threat," the statement read.

Junior defence minister Tobias Ellwood was more blunt, retweeting a message from Trump that the jihadists had been defeated in Syria with the words: "I strongly disagree.

"It has morphed into other forms of extremism and the threat is very much alive."

The Times newspaper on Thursday reported that Britain had not been informed of the decision before Trump announced it.

France said Thursday it will maintain its participation in the coalition fighting IS forces in Syria.

European Affairs Minister Nathalie Loiseau said "the fight against terrorism is not over."

A US presence in Syria is seen as key to pushing against Russian and Iranian influence. Pro-Iran militias have supported Syrian President Bashar al-Assad, and Moscow in 2015 intervened in the conflict to prop him up.

Charles Lister, a senior fellow at the Middle East Institute, called the decision "extraordinarily short-sighted and naive."

"This is not just a dream scenario for ISIS, but also for Russia, Iran and the Assad regime, all of whom stand to benefit substantially from a US withdrawal," Lister said.