The stunning move will have extraordinary geopolitical ramifications and throws into question the fate of US-backed Kurdish fighters who have been tackling Islamic State jihadists.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," the Republican president tweeted.

The US official said Trump's decision was finalized Tuesday.

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

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

The official would not provide a timeline for a withdrawal, saying only: "We will ensure force protection is adequately maintained, but as quickly as possible."

The Pentagon would not confirm the US troop pull-out.

"At this time, we continue to work by, with and through our partners in the region," Pentagon spokesman Colonel Rob Manning said.

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 and is viewed by Turkey as 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.

Ties have grown even more fraught since the US set up observation posts in northern Syria close to the border with Turkey to prevent any altercation between Turkish forces and the YPG.

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

- 'Short-sighted and naive'

The decision to withdraw marks a shocking development not just for Kurds in Syria, but for long-established US doctrine in the region.

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

"The military objective is the enduring defeat of ISIS. And if we've learned one thing over the years, enduring defeat of a group like this means you can't just defeat their physical space and then leave."

A US presence in Syria is seen as key to pushing against Iranian influence in the country and across the broader region. Tehran militias have supported the regime of President Bashar al-Assad.

Republican Senator Lindsey Graham, a Trump ally, said the president's decision was shortsighted.

"President @realDonaldTrump is right to want to contain Iranian expansion," Graham said on Twitter.

"However, withdrawal of our forces in Syria mightily undercuts that effort and put our allies, the Kurds at risk."

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

"This move will look like a 'withdrawal,' not a 'victory,' and yet more evidence of the dangerous unpredictability of the US president," Lister said.

"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."

IS swept across large swaths of Syria and neighboring Iraq in 2014, implementing their brutal interpretation of Islamic law in areas they controlled.

But they have since seen their dream of a state crumble, as they have lost most of that territory to various offensives.

In Syria, IS fighters are holding out in what remains of the pocket that once included Hajin, including the villages of Al-Shaafa and Sousa.