The US-led military coalition in Syria has begun the process of withdrawing troops, an American military official said.
Colonel Sean Ryan, spokesman for the coalition fighting the Islamic State group, said the US had started "the process of our deliberate withdrawal from Syria".
"Out of concern for operational security, we will not discuss specific timelines, locations or troop movements," he said in a statement.
The Britain-based Syrian Observatory for Human Rights, which monitors the conflict in Syria through a network of activists on the ground, said the withdrawal began on Thursday night.
It said a convoy of about 10 armoured vehicles and some trucks pulled out from Syria's north-eastern town of Rmeilan into Iraq.
Confirmation of the first withdrawals comes amid confusion over plans to implement President Donald Trump's pullout order and threats from Turkey to attack the Kurds, who have been America's partners on the ground in the war against IS in Syria.
There are 2,000 American troops in Syria. Mr Trump's abrupt decision in December to pull them out, declaring in a tweet the defeat of IS, sent shockwaves across the region and led to the resignation of US defence minister James Mattis and the top US envoy to the anti-IS coalition.
It also led to major criticism that the US was abandoning its Kurdish allies amid Turkish threats of an imminent attack.
On Sunday, US National Security Adviser, John Bolton, said American troops will not leave north-eastern Syria until IS is defeated and American-allied Kurdish fighters are protected.
Secretary of State Mike Pompeo, who is on a tour of the region, has also sought to reassure the Kurds that they will be safe after US troops withdraw from the country.
Mr Pompeo said:
Russia's Foreign Ministry said the Americans are not serious about withdrawing.
Spokeswoman Maria Zakharova said it appears to Moscow that the US "is looking for a reason to stay".
She said Russia has not seen public statements laying out the US strategy in Syria and so cannot be sure Washington is serious about leaving.
- Press Association