U.S. troops have begun withdrawing from positions in northern Syria, paving the way for a Turkish operation against Kurdish fighters in the border area, the BBC reports.

Kurdish-led forces have until now been a key U.S. 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 U.S. move a "stab in the back".

But President Donald Trump defended the pullout, saying it was time "to get out of these ridiculous Endless Wars".

The withdrawal follows a White House statement issued late on Sunday, saying U.S. troops were stepping aside for an imminent Turkish operation.

Turkish President Recep Tayyip Erdogan says its aim is to combat Kurdish fighters in the border area, and to set up a "safe zone" for Syrian refugees currently living in Turkey.

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.