According to the United States, it is stepping aside for an imminent Turkish operation against Kurdish-led forces within Syria that have until now been a key US ally.
Kurdish militias played a major role in defeating the Islamic State (IS) group, but Turkey regards them as terrorists.
The US - which has hundreds of troops in north-eastern Syria - has begun to withdraw them from a border area where Turkey seeks to set up a "safe zone".
Syria's main Kurdish-led group called the US move a "stab in the back".
In January, President Trump threatened to "devastate Turkey economically" if it attacked Kurdish forces.
"Turkey will soon be moving forward with its long-planned operation into northern Syria," the statement said.
"The United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the Isis territorial 'Caliphate', will no longer be in the immediate area."
The White House also said that Turkey would take over all responsibility for IS fighters captured by Kurdish forces over the past two years.