Colonel Sean Ryan, spokesman for the coalition fighting the so-called Islamic State (IS) 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.
An official said the equipment withdrawal is underway and an unspecified number of additional US troops have been moved into Syria to assist with the withdrawal process, including to provide additional security.
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.
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.
"These have been folks that have fought with us and it's important that we do everything we can to ensure that those folks that fought with us are protected," Mr Pompeo said of the Kurds while visiting Irbil, the capital of Iraq's semi-autonomous Kurdistan region, after talks in Baghdad.