US President Donald Trump on Monday threatened to destroy the economy of Nato ally Turkey if Ankara took a planned military strike in Syria too far, even though the US leader opened the door for an incursion.

Mr Trump said he would “obliterate” Turkey’s economy if it took action that he considered off-limits against America's Kurdish allies in the fight against ISIS, after his decision on Sunday to abandon them in north-eastern Syria.

But the Turkish Defence Ministry said on Twitter that all preparations for a military operation into north-eastern Syria were completed.

The ministry said that establishing a safe zone was essential for the stability and peace of the region and for Syrians to live in safety.

The US military said earlier on Monday that it was blocking Turkey from Syria’s north-eastern airspace.

US defence spokeswoman Carla Gleason said that Ankara was removed from the anti-ISIS coalition's air forces co-ordination system.

A US State Department official told CNN: “We have shut down Turkish participation in the air. Right now we are controlling the airspace over north-east Syria.”

“We have shut down Turkish participation in the air. Right now we are controlling the airspace over northeast Syria,” a senior @StateDept official says. “We have no intention in the immediate near future to change that" — Jennifer Hansler (@jmhansler) October 7, 2019

It is unclear if the US would act to block Turkish jets in the area, or was only seeking to distance itself from Ankara's operation against the Kurdish-led militias.

Mr Trump's move on Sunday marked a major shift in US policy and abandoned an ally in the battle against ISIS, which took over large areas of Syria.

On Monday, a senior US official appeared to contradict Mr Trump's decision, saying that US troops would be moved within the country to keep them out of harm's way from Turkey's expected incursion.

The official said the US moves in Syria, including the dismantling of two observation posts on Monday, are "not a beginning of a pullout".

This went against Mr Trump's tweet earlier in the day that the US would "get out" of the war.

The senior official described a safety plan that would move 50 US special operation forces away from the area that Turkey was looking to invade.

"They will move to more secure areas over the next several days," he said.

The official said Mr Trump was not giving approval to Turkey's incursion into Syria.

He said there would be no US support or involvement in the operation and that Turkey was "intent on resettling refugees" in the area it sought to control, as well as dealing with ISIS captives.

These are two goals that appeal to Mr Trump and his administration.

The Pentagon on Monday also said that the US government told Turkey it did not endorse an operation in northern Syria.

Assistant to the Secretary of Defence for public affairs, Jonathan Hoffman, said: “In conversations between the department and the Turkish military, we have consistently stressed that co-ordination and co-operation were the best path towards security in the area.”

But at the White House, Mr Trump was sending different signals.

After his call with Turkish President Recep Tayyip Erdogan on Sunday, he decided that: "The United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial ‘caliphate’, will no longer be in the immediate area.”

On Monday morning, a small contingent of US troops left two observation posts in Tal Abyad and Ein Eissa but the broader US military plans were unclear.

Mr Trump took to Twitter on Monday to again pledge full withdrawal from Syria.

“It is time for us to get out of these ridiculous endless wars,” he tweeted.

....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

At the State Department and the Pentagon, US officials to whom The National spoke were surprised by Mr Trump’s decision.

One said that the officials were scrapping and reviewing their plans to adjust to the president's about-face.

Nicholas Heras, a senior fellow at the Centre for New American Security, did not see a clear US plan for the situation.

“The US government is scrambling right now to make sense of Mr Trump's new policy,” Mr Heras told The National.

“There is no consensus how this will all unfold, except that Turkey has neither the willingness nor the capability to prevent ISIS from re-emerging.”

Democratic and Republican leaders in Congress, Nancy Pelosi and Mitch McConnell, opposed Mr Trump’s Sunday announcement and called on him to reverse it.

"Turkey will soon be moving forward with its long-planned operation into northern Syria," the White House said late on Sunday evening.

The National

Mr Trump followed Sunday's announcement with a five-tweet statement lamenting the amount of time the US intervention had taken.

"Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out, and what they want to do with the captured ISIS fighters in their 'neighbourhood'," he wrote.

Mr Trump criticised European allies for treating the US like a "sucker" over the return of foreign fighters to their home nations.

"Europe did not want them back. They said, 'You keep them, USA'," he wrote.

"I said, 'No, we did you a great favour and now you want us to hold them in US prisons at tremendous cost. They are yours for trials'."

Most foreign fighters captured in Syria are being held by the Syrian Defence Forces in camps such as Al Hol in the north-west of the country.

.....again said “NO,” thinking, as usual, that the U.S. is always the “sucker,” on NATO, on Trade, on everything. The Kurds fought with us, but were paid massive amounts of money and equipment to do so. They have been fighting Turkey for decades. I held off this fight for.... — Donald J. Trump (@realDonaldTrump) October 7, 2019

Turkey has long called for a 32-kilometre "safe zone" under its control to be established along the border to drive back the Kurdish YPG militia, the dominant force in the SDF alliance.

Ankara considers the militia to be a terrorist organisation and a threat to its national security.

The US backed the SDF to defeat ISIS militants in Syria, and has until now been seeking a joint "security mechanism" with Turkey along the border to meet Ankara’s security needs without threatening its allies.

Mr Trump's Sunday decision to withdraw from north-eastern Syria was met with criticism from Europe.

French Defence Minister Florence Parly said it could open the door to a revival of ISIS.

“We are going to be extremely careful that this announced disengagement from the United States and a possible offensive by Turkey does not create a dangerous manoeuvre that diverts from the goal we all pursue – the fight against ISIS – and which is dangerous for the local population,” Ms Parly said on Monday.

“We must be extremely vigilant that a manoeuvre of this kind cannot, contrary to the goal of the coalition, strengthen Daesh rather than weaken it and eradicate it."

On Sunday, Mr Erdogan and Mr Trump agreed to meet in Washington next month to discuss a safe zone in northern Syria, the Turkish presidency said.

Mr Erdogan also expressed his "frustration over the US military and security bureaucracy's failure" to enforce an August deal establishing the zone.

The day before, the Turkish leader said that Ankara could launch a cross-border offensive "as soon as today, tomorrow".

Ankara says the YPG is a "terrorist" offshoot of Kurdish insurgents in Turkey.

Turkey said it wanted to urgently establish the zone for some of the 3.6 million refugees who fled the war in Syria, amid a growing public backlash against their presence in Turkey.

But the UN said on Monday that it hoped any Turkish military operation would not lead to the displacement of civilians or atrocities.

Panos Moumtzis, UN humanitarian co-ordinator for the Syria crisis, said in Geneva that the body had drawn up contingency plans to provide aid.

"We are hoping for the best but preparing for the worst," Mr Moumtzis said.

He said the UN had seen a "bitter history" of safe zones in places such as Srebrenica, the site of a 1995 massacre of more than 8,000 Bosniaks.