US President Donald Trump has launched a harsh attack on NATO ally Turkey, threatening to destroy its economy if Ankara takes a planned military strike in Syria too far even though the US leader himself has opened the door for a Turkish incursion.

Trump said he would “totally destroy and obliterate” Turkey’s 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.

Local media are reporting Turkey have already conducted airstrikes in Syria against the Syrian Democratic Forces, although there are suggestions the airstrikes hit targets in Iraq.

The Jerusalem Post reports a White House official stated later on Monday night that the US has not seen any signs of a Turkish operation in Syria yet.

“It appears the Turks are intent on some sort of military operation, possibly combined with an effort to resettle refugees,” said the White House official.

Trump’s stern 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, 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!)” 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, 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. WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN. Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out.”

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.

US relations with Turkey under Trump have been rocky.

Last year, he imposed tariffs on Turkish steel and aluminium products over Ankara’s detention of a US pastor whose case was supported by members of his Christian conservative base.

US relations with Turkey under Trump have been rocky.

Last year, he imposed tariffs on Turkish steel and aluminium products over Ankara’s detention of a US pastor whose case was supported by members of his Christian conservative base.