US troops have begun withdrawing from positions in northern Syria, paving the way for a Turkish operation against Kurdish fighters in the border area.
Kurdish-led forces have until now been a key US ally in Syria, where they helped defeat the Islamic State group, but Turkey regards them as terrorists.
The main Kurdish-led group called the surprise US move a "stab in the back".
The UN humanitarian chief in Syria, Panos Moumtzis, says aid workers are "preparing for the worst" if fighting breaks out in north-eastern Syria.
"From experience, this could result in a displacement of people, we want to make sure that we are ready," he told reporters.
A deal made with the US continues to prove itself of roughly equivalent or lower value to the paper it is written on!