(CNN) In a remarkable announcement late Sunday night, the White House said that United States forces in northern Syria would move aside in advance of a planned Turkish military offensive.

The move marks a major shift in US foreign policy and effectively gives Turkey the green light to attack US-backed Kurdish forces. The group, long considered as among Washington's most reliable partners in Syria, has played a key strategic role in the campaign against ISIS in the region.

Following a phone call between US President Donald Trump and Turkish President Recep Tayyip Erdogan, the White House said that Turkey would soon begin a military offensive and US forces would not be involved in the operation.

"Turkey will soon be moving forward with its long-planned operation into Northern Syria," a statement said. "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."

The White House added Turkey would now be responsible for all captured ISIS fighters who are currently being held by US-backed Kurdish forces in northern Syria.

As of last month, the US said about 1,000 US troops were operating in northeastern Syria. Sunday's statement did not specify if this constituted a full withdrawal of personnel from the country.

Presients Trump and Erdogan had a phone call Sunday where they discussed Turkey's military operation into northern Syria.

'Peace efforts'

Erdogan confirmed Monday that US troops had begun withdrawing from northeast Syria. He announced Saturday that Turkey had "completed our preparations and action plan" and was ready to launch a "ground and air operation" east of the Euphrates river, with the goal of establishing "peace" by clearing the region of "terrorists."

Turkey's operation is aimed at clearing the US-backed Kurdish militia -- the People's Protection Units (YPG) -- away from Turkey's border.

Ankara regards the YPG as a terrorist group affiliated with the Kurdistan Workers' Party (PKK), which has fought the Turkish state for more than three decades. But the US backs the YPG and credits the Kurds for helping defeat ISIS in Syria.

Turkey has been working with the US to establish "safe zones" that run along the Turkey-Syria border, but Anadolu reports that while Turkey welcomes the joint patrols, it has also said the US is not doing enough to set up the safe zone properly.

In a phone call with Trump, Erdogan expressed frustration over the US military's failure to implement the agreement between the two nations, according to a readout of the call released by the Turkish Presidency. The two leaders agreed to meet in Washington next month on Trump's invitation.

The US and Turkey have been working to establish the buffer zone, which the US calls a "security mechanism," in northeast Syria as part of a bid to prevent a military incursion into the area that would target Syrian Kurdish groups, a potential operation that the US fears could undermine the fight against ISIS.

The US military's involvement along the Syria-Turkish border includes manning observation posts as well as patrolling with Turkish troops in the area.

US changes tone

Trump's decision to allow Erdogan to move forward with the operation and to move US forces out of the area goes against efforts by US officials to dissuade Turkey from carrying out a military intervention.

"Any uncoordinated military operation by Turkey would be of grave concern as it would undermine our shared interest of a secure northeast Syria and the enduring defeat of ISIS," Pentagon spokesman Sean Robertson said recently.

He added that the US was committed to implementing the security mechanism, or safe zone, and contrary to Erdogan's comments, said it was "on time, or ahead of schedule, in many areas."

On Monday, a Pentagon spokesperson said the Department of Defense did not endorse the planned operation.

"The Department of Defense made clear to Turkey -- as did the President -- that we do not endorse a Turkish operation in Northern Syria. The US Armed Forces will not support or be involved in any such operation," Pentagon spokesman Jonathan Hoffman said in a statement

On Saturday, US Secretary of State Mike Pompeo told reporters that "we've made clear that this conflict shouldn't be militarized."

However, a US official familiar with operations in Syria told CNN that is "very likely" Turkey will try something soon. The official said it's possible Ankara will start with a limited incursion to establish Turkish patrol bases on the Syrian side of the border with the goal of launching more ambitious clearance operations later.

"A response by the Syrian Democratic Forces (SDF) would play into Turkey's narrative that they need to make the area safe to return refugees."

From the Kurdish-led SDF's perspective they feel that any unilateral action by Turkey on Syrian territory is a threat and they must respond.

"Allowing Turkish patrol bases now opens the door to permanent occupation and makes it easier for the Turkish military to attack the SDF later," the official said.

On Monday some Republican politicians and former Trump administration officials criticized the President's decision to "abandon" Kurdish allies.

US Senator Lindsey Graham tweeted : "This decision to abandon our Kurdish allies and turn Syria over to Russia, Iran, & Turkey will put every radical Islamist on steroids. Shot in the arm to the bad guys. Devastating for the good guys."

Brett McGurk, the former US envoy to the anti-ISIS coalition, heavily ripped into Trump, saying he was "not a Commander-in-Chief" and "makes impulsive decisions with no knowledge or deliberation."

"He sends military personnel into harm's way with no backing," McGurk tweeted . "He blusters and then leaves our allies exposed when adversaries call his bluff or he confronts a hard phone call."

And former US ambassador to the United Nations Nikki Haley reiterated on Twitter that Kurdish fighters "were instrumental in our successful fight against ISIS in Syria," adding that the US "must always have the back of our allies, if we expect them to have our back."

In response to criticism, Trump said on Twitter that "if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate (its) economy."

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

SDF pull fighters from guarding ISIS prisoners

The SDF announced Monday that it was pulling some of their fighters from guarding captured ISIS prisoners and was sending them towards the Syrian-Turkish border.

"We were doing our best to provide the best kind of security in the prisons and in the camps... [but] with the Turkish invasion... we are forced to pull out some of our troops from the prisons and from the camps to the border to protect our people," SDF spokesman Mustafa Bali told CNN.

This comes as a US official confirmed to CNN that the SDF had been informed that the US would not defend them from Turkey.

The SDF released a scathing statement Monday, accusing the US of "not abiding by their commitments" to avoid a military escalation.

"This Turkish military operation in northern and eastern Syria will have a large, negative impact on our war against ISIS and will destroy all the stability that has been accomplished over the past years," the SDF statement said.

"We in the Syrian Democratic Forces will not hesitate for a moment to defend ourselves," it continued, adding that it would call upon its allies to help defend from "Turkish aggression."

A spokesman for Syria's Manbij Military Council (MMC) -- which is a coalition of SDF groups -- also criticized Trump's decision to withdraw troops from northern and eastern Syria.

Shervan Darwish said it "will give a morale boost to ISIS's sleeper sells and will create a vacuum that ISIS will certainly use."

However Turkey's presidential spokesperson Ibrahim Kalin tweeted Monday that the two aims of the safe zones in Syria are to clear "terror elements to ensure the safety of our border and the safe return of refugees."

Kalin also tweeted that "we have no interest on anybody's territory."

Turkey plans to resettle two million Syrians in a 30-kilometer-wide (18.6 miles) safe zone to be set up in Syria, stretching from the Euphrates River to the Iraqi border, including Manbij, according to Anadolu.