President Donald Trump has threatened to "destroy" Turkey's economy if it takes a planned military strike in Syria too far, even though the US leader himself has opened the door for a Turkish incursion.
Mr Trump said he would "obliterate" its economy if it took action in Syria that he considered "off-limits" following his decision on Sunday to pull out US forces from northeastern Syria.
The US withdrawal will leave Kurdish-led forces in Syria that have long allied with Washington vulnerable to a planned incursion by the Turkish military which brands them terrorists.
A senior Trump administration official said on Monday that 50 American troops in the region that Turkey has targeted would be redeployed elsewhere in Syria.
The official said Mr Trump's decision to redeploy the troops does not constitute a US withdrawal from Syria.
Mr Trump's words seemed to be aimed at placating critics who accused him of abandoning the Syrian Kurds by pulling out US forces.
Leaders from both parties and both houses of Congress joined in the criticism, including Senate Majority Leader Mitch McConnell, Mr Trump's fellow Republican.
"As I have stated strongly before, and just to reiterate, if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I've done before!)" Mr Trump tweeted.
Turkey's lira slid more than 2 per cent to its lowest level in more than a month against the dollar on Monday over concerns about the planned incursion into northern Syria and Trump's warning.
Investors have been closely watching tense ties between Ankara and Washington in recent months, with the countries at odds over a range of issues, including Syria and Turkey's purchase of Russian missile defence systems.
Turkey has repeatedly threatened to carry out an incursion against US-backed Kurdish fighters in northeastern Syria who have links to Kurdish guerrillas operating next door in Turkey.
The United States began pulling troops back from the northeast Syrian border on Monday, effectively giving Turkey a green light to move into the area.
Earlier on Monday, Mr Trump said the United States should leave others from European allies to Iranian foes, "to figure the situation out" in the region.
He wrote on Twitter that "it is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home."
It is a major policy shift that was denounced as a "stab in the back" by Kurdish-led forces who have been Washington's most capable partners in fighting Islamic State in Syria, also known by its acronym ISIS.
The United States expects Turkey to take responsibility for captive Islamic State fighters in northeastern Syria if Ankara's planned incursion seizes areas where the detained militants are held, a senior State Department official said.
The captives are held in SDF facilities south of a safe zone initially proposed by Turkey.
Aside from Trump's threat, the State Department official and the Pentagon both said the United States did not endorse Turkey's planned offensive.
"We made it clear (to the Turks) that we do not support this operation," the official told reporters. "We think this operation is a very bad idea."
A US official said Turkey had been removed from a military mechanism used to coordinate air operations over northern Syria and that Turkey would no longer have access to US intelligence and surveillance feeds in the region.