US President Donald Trump ordered the withdrawal of US troops from Turkey's border with Syria on Sunday 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 US 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 went too far.

Turkish Vice-President Fuat Oktay responded to Trump's threat on Tuesday, warning that "Turkey is not a country that will act according to threats".

"As our president always stresses, Turkey will always set its own path and will take matters into its own hands," Oktay said in a speech in Ankara.

Turkey says it wants a "safe zone" along northern Syria to 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.