(BBC) -- The Trump administration says US troops are being withdrawn from Syria, after the president said the Islamic State (IS) group had been "defeated".
The Pentagon said it was transitioning to the "next phase of the campaign" but did not give details.
Some 2,000 troops have helped rid much of north-eastern Syria of IS, but pockets of fighters remain.
It had been thought defence officials wanted to maintain a US presence to ensure IS did not rebuild.
There are also fears a US withd
Both the Pentagon and the White House statement said the US had started "returning United States troops home as we transition to the next phase of this campaign".
The Pentagon said it would not provide further details of what that next phase is "for force protection and operational security reasons".
The White House said the US and its allies stood "ready to re-engage at all levels to defend American interests whenever necessary, and we will continue to work together to deny radical Islamist terrorists territory, funding, support and any means of infiltrating our borders".
President Trump said it was time to bring troops home after their "historic victories".
What has been the reaction?
Israel said it had been told the US had "other ways to have influence in the area" but would "study the timeline [of the withdrawal], how it will be done and of course the implications for us".
Russia's Foreign Ministry spokeswoman Maria Zakharova said on state-controlled Channel One TV that the US decision could result in "genuine, real prospects for a political settlement" in Syria.
Pulling troops out of Syria had long been promised by President Trump.
But the announcement may have taken some of his own officials by surprise.
Last week Brett McGurk, special presidential envoy for the global coalition to defeat IS, told reporters at the state department: "Nobody is saying that [IS fighters] are going to disappear. Nobody is that naive. So we want to stay on the ground and make sure that stability can be maintained in these areas."
The state department abruptly cancelled Wednesday's daily briefing after the withdrawal was announced.
One of Mr Trump's supporters, Republican Senator Lindsey Graham, who sits on the armed services committee, called it a "huge Obama-like mistake".
In a series of tweets, he said IS was "not defeated", and warned withdrawing US troops puts "our allies, the Kurds, at risk".
Turkey this week said it was preparing to launch an operation against a Kurdish militia in northern Syria, which has been an ally of the US in its fight against IS.
A UK government spokesman said these developments "do not signal the end of the Global Coalition or its campaign" against IS, and the UK will "remain committed" to ensure IS' "enduring defeat".
"Much remains to be done and we must not lose sight of the threat they pose," the UK statement said.
"As the situation on the ground develops, we will continue to discuss how we achieve these aims with our Coalition partners, including the US."