Turkey said Tuesday it was ready for an offensive into northern Syria, while President Donald Trump insisted the United States had not abandoned its Kurdish allies who would be targeted in the assault.
Trump has blown hot and cold since a surprise announcement on Sunday that Washington was pulling back 50 to 100 "special operators" from Syria's northern frontier.
The troops had served as a buffer preventing a long-planned attack by the Turkish military against Kurdish forces, who were crucial in the years-long campaign to defeat the Islamic State group but are viewed as "terrorists" by Ankara.
After appearing to give a green light to the Turkish invasion on Sunday, Trump later threatened to "obliterate" Turkey's economy if it went too far.
"Any unforced or unnecessary fighting by Turkey will be devastating to their economy and to their very fragile currency," he tweeted.
But he also gave a warm account of Turkey in other tweets and announced that President Recep Tayyip Erdogan would visit Washington on November 13.
"So many people conveniently forget that Turkey is a big trading partner of the United States," he said.
Ankara had already brushed aside Trump's warnings, with Vice-President Fuat Oktay saying: "Turkey is not a country that will act according to threats."
Turkey has always pushed hard against US support for Kurdish forces in Syria due to their links with the outlawed Kurdistan Workers' Party (PKK) which has fought a bloody insurgency against the Turkish state since 1984.
Its defence ministry tweeted that preparations for an offensive in northern Syria have been "completed".
Meanwhile, the Syrian government has welcomed the upheaval, spying an opportunity to bring the country's Kurds back into its fold.
The Kurds have been "tossed aside" by Washington, Syrian Deputy Foreign Minister Faisal Mekdad told the pro-government Al-Watan newspaper.
Trump rejected that interpretation, tweeting: "We may be in the process of leaving Syria, but in no way have we Abandoned the Kurds, who are special people and wonderful fighters."
- 'Deeply concerned' -
Britain said it was "deeply concerned" by Turkey's plans to attack Kurdish fighters, who lead the Syrian Democratic Forces (SDF) and say they lost some 11,000 fighters as the main frontline force against the Islamic State group.
A spokesman for Prime Minister Boris Johnson said London had been "consistently clear with Turkey that unilateral military action must be avoided as it would destabilise the region".
Iran, a key backer of the Syrian government, also opposed a Turkish incursion, with Foreign Minister Mohammad Javad Zarif urging "respect for Syria's territorial integrity" in a call to Ankara.
Turkey says it wants to establish a "safe zone" on the Syrian side of the border where it could send back some of the 3.6 million refugees from the eight-year civil war.
Erdogan said Monday that the operation into Syria could "come any night without warning."
Kurds argue that Ankara's goal is to dilute their dominance in the region with an influx of mostly Sunni Arab refugees now living in Turkey.
Trump has faced a barrage of criticism, including from close allies in Washington, for appearing to leave the Kurds to their fate.
Many officials were caught off-guard, with Pentagon spokesman Jonathan Hoffman warning Turkey of destabilising blowback to the region if it invades.
"The Department of Defense made clear to Turkey -- as did the president -- that we do not endorse a Turkish operation in Northern Syria," he said on Monday.
A Turkish attack also raises the specter of what will happen to some 10,000 Islamic State fighters currently being held in Kurdish detention centres.
Around 2,000 of them are IS "foreign fighters", and Trump assailed US allies in Europe for not taking back their nationals.
If they escape or are released, they could reconstitute the Islamic State, less than one year after it was defeated and its "caliphate" disintegrated.
Trump declared that it would be the responsibility of Turkey and other countries to deal with the IS prisoners.