US President Donald Trump ordered the withdrawal of US troops from Turkey's border with Syria after a phone call with his counterpart Recep Tayyip Erdogan.

The Turkish president has repeatedly threatened to attack Kurdish militants in northern Syria due to their ties with separatists in his own country.

"All of the preparations for an operation have been completed," the Turkish defence ministry tweeted, hours after US forces withdrew from the border area.

Erdogan had earlier said the operation could come at any moment "without warning".

Trump's move was seen by critics as an abandonment of Kurdish forces which had been the key ally in the fight against the Islamic State (IS) group.

But there was confusion later on Monday when Trump tweeted that he would "obliterate" Turkey's economy if it did anything he did not like.

The White House also said Turkey would be responsible for thousands of IS prisoners currently held in Kurdish detention centres.

Ankara says it wants a "safe zone" along northern Syria that will act as a buffer against Kurdish forces and also allow the return home of up to two million Syrian refugees.

It has previously launched two cross-border offensives against IS in 2016 and the YPG in 2018, with the support of Syrian rebels.