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 has condemned the pullout and accused the US of abandoning its fighters.
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.