The US will not stand in the way of a planned Turkish military operation against Kurdish fighters in northeast Syria, the White House has announced.

The surprise policy shift signals an abandonment by Washington of its longtime ally in the fight against Isis, the Kurdish-led Syrian Democratic Forces (SDF) and raises the prospect of a new conflict in the country’s multifaceted war.

It came late on Sunday evening following a phone call between Donald Trump and his Turkish counterpart Recep Tayyip Erdogan.

“Turkey will soon be moving forward with its long-planned operation into northern Syria. The United States armed forces will not support or be involved in the operation, and the United States forces, having defeated the Isis territorial ‘caliphate’, will no longer be in the immediate area,” the White House statement said.

Mr Trump, in a lengthy series of messages posted on Twitter early Monday, said the US deployment of troops to northern Syria was too costly, and Syria’s Kurds would now have to go it alone.

“The Kurds fought with us, but were paid massive amounts of money and equipment to do so. They have been fighting Turkey for decades,” Mr Trump said.

“Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out,” he added. “We are 7,000 miles away and will crush Isis again if they come anywhere near us!”

Timeline of the Isis caliphate Show all 19 1 /19 Timeline of the Isis caliphate Timeline of the Isis caliphate ISIS began as a group by the merging of extremist organisations ISI and al-Nusra in 2013. Following clashes, Syrian rebels captured the ISIS headquarters in Aleppo in January 2014 (pictured) AFP/Getty Timeline of the Isis caliphate Abu Bakr Al-Baghdadi declared the creation of a caliphate in Mosul on 27 June 2014 Timeline of the Isis caliphate Isis conquered the Kurdish towns of Sinjar and Zumar in August 2014, forcing thousands of civilians to flee their homes. Pictured are a group of Yazidi Kurds who have fled Rex Timeline of the Isis caliphate On September 2 2014 Isis released a video depicting the beheading of US journalist Steven Sotloff. On September 13 they released another video showing the execution of British aid worker David Haines Timeline of the Isis caliphate The US launched its first airstrikes against Isis in Syria on 23 September 2014. Here Lt Gen William C Mayville Jnr speaks about the bombing campaign in the wake of the first strikes Getty Timeline of the Isis caliphate Isis militants sit atop a hill planted with their flag in the Syrian town of Kobani on 6 October 2014. They had been advancing on Kobani since mid-September and by now was in control of the city’s entrance and exit points AFP/Getty Timeline of the Isis caliphate Residents of the border village of Alizar keep guard day and night as they wait in fear of mortar fire from Isis who have occupied the nearby city of Kobani Getty Timeline of the Isis caliphate Smoke rises following a US airstrike on Kobani, 28 October 2014 AFP/Getty Timeline of the Isis caliphate YPG fighters raise a flag as they reclaim Kobani on 26 January 2015 VOA Timeline of the Isis caliphate Isis seized the ancient Syrian city of Palmyra on 20 May 2015. This image show the city from above days after its capture by Isis Getty Timeline of the Isis caliphate Kurdish forces are stationed on a hill above the town of Sinjar as smoke rises following US airstrikes on 12 November 2015 AFP/Getty Timeline of the Isis caliphate Kurdish forces enter Sinjar after seizing it from Isis control on 13 November 2015 AFP/Getty Timeline of the Isis caliphate Iraqi government forces make the victory sign as they retake the city of Fallujah from ISIS on 26 June 2016 Getty Timeline of the Isis caliphate Iraqi forces battle with Isis for the city of Mosul on 30 June 2017 AFP/Getty Timeline of the Isis caliphate Members of the Iraqi federal police raise flags in Mosul on 8 July 2017. On the following day, Iraqi prime minister Haider Al Abadi declares victory over Isis in Mosul Getty Timeline of the Isis caliphate Members of Syrian Democratic Forces celebrate in Al-Naim square after taking back the city of Raqqa from Isis. US-backed Syrian forces declare victory over Isis in Raqqa on 20 October 2017 after a four-month long campaign Getty Timeline of the Isis caliphate Female fighters of the Syrian Democratic Forces celebrate in Al-Naim Square after taking back the city of Raqqa from Isis. US-backed Syrian forces declare victory over Isis in Raqqa on 20 October 2017 after a four-month long campaign AFP/Getty Timeline of the Isis caliphate Trucks full of women and children arrive from the last Isis-held areas in Deir ez-Zor, Syria in January 2019 They were among the last civilians to be living in the ISIS caliphate, by this time reduced to just two small villages in Syria’s Deir ez-Zor Richard Hall/The Independent Timeline of the Isis caliphate Zikia Ibrahim, 28, with her two-year-old son and 8-month-old daughter, after fleeing the Isis caliphate, on Saturday 26 January 2019 Richard Hall/The Independent

The announcement appeared to have come directly from the White House, with no coordination with US forces in Syria or local allies. Washington’s partners in the coalition to defeat Isis, of which Britain is one, were not given advance warning of the decision, western diplomats told The Independent.

The policy shift has drawn criticism from both home and abroad. One of Mr Trump’s top allies, Republican senator Lindsey Graham, said he would introduce a Senate resolution to reverse the decision and punish Turkey should it attack the SDF.

“We will introduce bipartisan sanctions against Turkey if they invade Syria and will call for their suspension from Nato if they attack Kurdish forces who assisted the US in the destruction of the Isis caliphate,” he wrote on Twitter.

“It’s never in our national security interest to abandon an ally who’s helped us fight Isis,” Mr Graham said in an interview with Fox News Channel. “This impulsive decision by the president has undone all the gains we’ve made, thrown the region into further chaos.”

European Commission spokesperson Maja Kocijancic said: “Renewed armed hostilities in the northeast will not only exacerbate civilian suffering and lead to massive displacement but will also risk severely undermining current political efforts.”

The SDF, a mostly Kurdish militia with some smaller Arab groups attached, has been Washington’s main ally in the fight against Isis in Syria. The group received US arms, as well as significant air and ground support in the battle to defeat the Isis caliphate. It lost some 11,000 fighters in the years-long war before recapturing the last piece of Isis territory in March.

Syrian Kurds visit the tombs fighters who lost their lives fighting Isis (AFP/Getty)

Since the victory over Isis, the SDF has continued to working alongside US forces to destroy sleeper cells across its areas of control. The group is also holding thousands of Isis prisoners, and tens of thousands of their family members.

The alliance has always infuriated Ankara, which considers the SDF a terrorist organisation for its links to banned Kurdish separatist organisation inside Turkey. It has pressured the US to drop its commitment to the group and allow it to take responsibility for ensuring the complete defeat of Isis.

Mr Trump has previously made abrupt announcements regarding the fate of US forces in Syria, only to later reverse them. But the SDF said on Monday that US forces had already begun to withdraw from the area along Turkey’s border.

“The American forces did not fulfil their commitments and withdrew their forces from the border areas with Turkey, and Turkey is now preparing for an invasion operation of northern and eastern Syria. Erdogan’s threats are aimed to change the security mechanism into a mechanism of death, displace our people and change the stable and secure region into a zone of conflict and permanent war,” said a statement from the group.

“We are determined to defend our land at all costs,” it added.

SDF spokesperson Kino Gabriel said the group had been “stabbed in the back” by the US. In an interview on al-Hadath TV he said there had been “assurances from the United States of America that it would not allow any Turkish military operations against the region”.

The US currently has around 1,000 troops in northeast Syria, ostensibly to support Kurdish forces and ensure the lasting defeat of Isis. Their presence has also acted as a powerful deterrent for an attack by Turkey, which would not want to harm US soldiers.

Ankara has repeatedly threatened to enter Syria to implement a “safe zone” along the border, which it says is necessary for its own security. In September, following months of Turkish pressure, the SDF agreed to withdraw from some areas along the border to allow for joint US-Turkey patrols.

As part of the initial stages of the safe zone, the US oversaw the removal of SDF fortifications along the border, which were built to protect against a possible Turkish invasion. The SDF said it complied with the removals because it had anticipated the US would prevent an attack. But Turkey expressed frustration that the measures do not go far enough, and continued to threaten a unilateral operation.

President Erdogan said on Monday that he would travel to the US in the first half of next month to meet with Mr Trump, a sign that an operation was not imminent.

“Having a face-to-face meeting will give us the opportunity to discuss the depth of the operation, Turkey-US relations and developments in the region,” he said, adding that he hoped to also discuss the stalled F-35 fighter jet program.

Responding to the White House announcement, Turkey’s foreign minister Mevlut Cavusoglu sai​d Turkey would “contribute to peace and stability in Syria”.

“Since the beginning of the crisis in Syria, we supported the country’s territorial integrity, and we will continue to do so from now on,” he said in a tweet.

However, there was caution inside Turkey at the announcement. Yusuf Erim, a political and strategic analyst at Turkey’s TRT public broadcaster, said that it was too early to take the White House readout verbatim. Turkish officials are closely watching developments in Washington for signs of resistance within the US military and diplomatic corps to the US announcement.

“The next 24 hours and the reaction from the US political establishment and how much they’re going to back this Trump-Erdogan phone call is key,” he said.

Still, the news heartened the leadership in Ankara. Turkey has long viewed the Kurdish-led governance experiment in northeast Syria as a major threat to its security. Though it has been threatening to launch a military incursion into Rojava for months, it has built up military forces along the border in recent days.

“It’s a unilateral operation with a bilateral agreement. The US is saying, ‘I won’t help you but I won’t stand in the way,’” said Mr Erim. “Ankara is viewing this is a positive. Its main concern was any type of confrontation with American troops. We can see that America will be withdrawing its troops from the immediate area, which will take this risk off the table.”

A potential conflict between the SDF and Turkey would likely destabilise a region that has been relatively calm in the past few years. The SDF has warned that a Turkish attack would create a new refugee crisis in the region, and threaten the security of prisons holding thousands of Isis fighters.

The White House statement said that Turkey “will now be responsible for all Isis fighters in the area captured over the past two years in the wake of the territorial defeat of the ‘caliphate’ by the United States”. But it is unclear how any handover between the SDF and Turkey would work in the event of a conflict between the two.

Perhaps the harshest criticism of Mr Trump’s decision came from Brett McGurk, who resigned last year as the Trump administration’s special envoy on Isis, following a hastily announced troop withdrawal.

“Donald Trump is not a commander-in-chief. He makes impulsive decisions with no knowledge or deliberation. He sends military personnel into harm’s way with no backing. He blusters and then leaves our allies exposed when adversaries call his bluff or he confronts a hard phone call,” Mr McGurk said on Twitter.

He added that the White House statement “demonstrates a complete lack of understanding of anything happening on the ground. The ‘United States’ is not holding any Isis detainees. They are all being held by the SDF, which Trump just served up to Turkey.”

The UN said it was already preparing to deal with the fallout from any potential fighting.

“We are hoping for the best but preparing for the worst,” said Panos Moumtzis, UN regional humanitarian coordinator for the Syria crisis, adding that the UN had seen a “bitter history” of safe zones in places like Srebrenica.