The US says 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 border areas.
The main Kurdish-led force in Syria described the surprise announcement as a "stab in the back" by the US.
In January, President Trump threatened to "devastate Turkey economically" if it attacked Kurdish forces.
However, a White House statement issued on Sunday makes no reference to the Kurdish fighters.
The statement followed a phone call between President Donald Trump and Turkish President Recep Tayyip Erdogan.
What did the White House say?
"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.More than 12,000 men are held on suspicion of being IS members in Kurdish-controlled camps located south of the Turkey's planned "safe zone". At least 4,000 of them are foreign nationals./BBC news