The Trump administration says US troops are being withdrawn from Syria, after the president said the Islamic State (IS) group had been "defeated".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 withdrawal will cede influence in Syria and the wider region to Russia and Iran.
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"