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.

Mr Trump said he – in his “great and unmatched wisdom” – 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 north-eastern 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 has already conducted airstrikes along the Syria-Iraq border against the Syrian Democratic Forces.

The Jerusalem Post reports a White House official stated later on Monday that the US had seen no 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,” the White House official said.

Videos have appeared on social media purporting to show Turkish attacks, just hours after the US announced it would be pulling out of the region.

Turkish planes reportedly attacked SDF headquarters in Hasakah province, according to Saudi broadcaster al-Ikhbariya. Other reports suggest Turkey targeted the Kurdish YPG militia, which Ankara views as a terrorist group and which is part of the SDF.

#Syria: video showing Turkish Air Force targeting area of Semalka border crossing with #Iraq (used to transfer supplies). https://t.co/SAUkTWnbec pic.twitter.com/LLU9zjCzwR

— Qalaat Al Mudiq (@QalaatAlMudiq) October 7, 2019

Turkish president Recep Tayyip Erdogan said on Tuesday that Ankara was ready to launch an assault against Kurdish troops “at any moment”.

“We have made a decision. We said ‘one night we could come suddenly’. We continue our determination,” he said as American forces began pulling out of Syria’s war-ravaged north-east.

Mr Trump’s stern words seemed to be aimed at placating critics who have accused him of abandoning the Kurds with the US withdrawal.

Leaders from both parties and both houses of Congress joined in the criticism, including Senate Majority Leader Mitch McConnell, Mr Trump’s fellow Republican.

Mr Trump’s decision to effectively abandon the US’s Kurdish allies has been roundly criticised by some members of his own party.

“A catastrophic mistake,” House Republican Conference Chair Liz Cheney said, while Senator Lindsey Graham called it a “shot in the arm to the bad guys”.

“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.

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!). They must, with Europe and others, watch over…

— Donald J. Trump (@realDonaldTrump) October 7, 2019

Turkey’s lira slid more than 2 per cent to its lowest level in more than a month against the US dollar on Monday amid concerns about the planned incursion into northern Syria and Mr 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 north-eastern Syria, who have links to Kurdish guerrillas operating next door in Turkey.

The United States began pulling troops back from the north-east Syrian border on Monday, effectively giving Turkey a green light to move into the area.

Earlier on Monday, Mr Trump said the US 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.”

….almost 3 years, but 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…..

— Donald J. Trump (@realDonaldTrump) October 7, 2019

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 US expects Turkey to take responsibility for captive Islamic State fighters in north-eastern 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 Mr Trump’s threat, the State Department official and the Pentagon both said the US did not endorse Turkey’s planned offensive.

“We made it clear [to the Turks] that we do not support this operation,” the official said. “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.

WATCH: US forces begin withdrawal from Syrian-Turkey border as Turkey prepares to launch a long-planned operation in Kurdish-held areas in northeast Syria. https://t.co/bgHeeMOa0A pic.twitter.com/iCIe9Wuhht

— NBC News (@NBCNews) October 7, 2019

US relations with Turkey under Mr 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.

-with AAP