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.