Update: Trump warns Turkey over Syria: I will obliterate your economy if you do anything off limits | Lindsey Graham slams Trump for Syria withdrawal | Trump in Syria tweet storm: U.S. getting out of 'ridiculous endless wars'

American troops began withdrawing Monday from their positions along Turkey's border in northeastern Syria, in a major shift in U.S. policy harshly criticized by its Kurdish allies, which came despite Pentagon officials' support for maintaining American presence in the region.

The partial pullout comes ahead of an anticipated Turkish invasion, which U.S. President Donald Trump endorsed in a Sunday phone call with Turkish President Recep Tayyip Erdogan and that U.S.-backed Kurdish-led forces say will overturn five years of achievements in the battle against the Islamic State group.

"Despite our efforts to avoid any military escalation with Turkey and the flexibility we have shown to move forward in establishing a mechanism for the security of the borders ..., the American forces did not fulfill their commitments and withdrew their forces from the border areas with Turkey," U.S-backed Kurdish-led Syrian Democratic Forces, which controls most of the area, said in a statement.

"We will not hesitate for a moment in defending our people" against Turkish troops, the Syrian Kurdish force said, adding that it has lost 11,000 fighters in the war against ISIS in Syria.

"There were assurances from the United States of America that it would not allow any Turkish military operations against the region," SDF spokesman Kino Gabriel said in an interview with al-Hadath TV on Monday.

The SDF had been "completely committed" to a U.S.-guaranteed deal for a "security mechanism" for the border area, he added. "But the (U.S.) statement today was a surprise and we can say that it is a stab in the back for the SDF," he said.

Open gallery view U.S. troops walk past a Turkish military vehicle during a joint patrol with Turkish troops in the Syrian village of al-Hashisha, September 8, 2019. Credit: Delil Souleiman/AFP

A U.S. official told Reuters the United States informed the commander of the SDF forces on Monday morning that it will not defend them from Turkish attacks anywhere.

Abdulkarim Omar, who acts as foreign minister for the Syrian Kurds, said on Monday the White House statement about transferring to Turkey the responsibility for thousands of foreign ISIS fighters held in northeastern Syria was "illogical."

Omar said the U.S. troop withdrawal from the border will have "catastrophic consequences" because Kurdish-led forces would be preoccupied with defending the border, instead of protecting detention facilities or the crowded al-Hol camp which houses over 73,000 people, many of them ISIS families and supporters.

Omar called on the international community to work to reverse Trump's decision or stop the Turkish offensive.

The American withdrawal, confirmed by U.S., Kurdish and Turkish officials and Syrian rebel groups, came just hours after the White House said U.S. forces in northeastern Syria will move aside and clear the way for an expected Turkish assault — essentially abandoning Kurdish fighters who fought alongside American forces in the yearslong battle to defeat ISIS.

The White House said late on Sunday Turkey will soon move forward with its long-planned military operation there to create what it calls a "safe zone," adding U.S. forces would not support or be involved in it.

The American statement came after Erdogan and Trump discussed in a phone call Turkey's plans to establish a "safe zone" east of the Euphrates River in Syria.

Open gallery view Syrian Kurds demonstrate against Turkish threats in the town of Ras al-Ain in Syria's Hasakeh province near the Turkish border, October 6, 2019. Credit: Delil Souleiman/AFP

Trump's decision to back the Turkish operation defies the stance of senior U.S. defense officials, who sought to keep a small presence in northern Syria to combat ISIS, and to counter the influence of both Russia and Iran in the region, the New York Times reported on Monday.

Republicans and Democrats have warned that allowing the Turkish attack could lead to a massacre of the Kurds and send a troubling message to American allies across the globe.

The Turkish presidency said after the call that Erdogan and Trump had agreed to meet in Washington next month, following an invitation by the U.S. president.

Erdogan's spokesman said the planned "safe zone" in northern Syria aims to clear terrorist elements from the border and return refugees safely to Syria within the framework of Syrian territorial integrity.

Turkey is determined to clear its border with Syria of militants and assure the security of the country, Foreign Minister Mevlut Cavusoglu said on Monday.

"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," Cavusoglu said. "We will contribute to peace and stability in Syria," he said in a tweet.

Open gallery view Syrian Kurds gather around a U.S. armored vehicle during a demonstration against Turkish threats next to a U.S.-led international coalition base on the outskirts of Ras al-Ain, Syria, October 6, 2019. Credit: Delil Souleiman/AFP

The call came a day after Erdogan said a military incursion into northeastern Syria was imminent, after Ankara accused Washington of stalling efforts to establish a "safe zone" there together.

During the phone call, Erdogan expressed his frustration with the failure of U.S. military and security officials to implement the agreement between the two countries, the Turkish presidency said.

Erdogan also reiterated the necessity of the safe zone to eliminate the threats from the Syrian Kurdish YPG militia, which Ankara considers a terrorist organisation, and to create the conditions necessary for the return of Syrian refugees, it said.

"Turkey will soon be moving forward with its long-planned operation into Northern Syria," the White House Press Secretary said in a statement .

"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," it added.

The statement from the White House also said "Turkey will now be responsible for all ISIS fighters in the area captured over the past two years", as France, Germany and other European nations that they had come from had refused U.S. requests to take them back.

The NATO allies agreed in August to establish a zone in northeast Syria along the border with Turkey. Ankara says the zone should be cleared of the YPG.

However, Kurdish group YPG said a Turkish attack would lead to a resurgence of ISIS, which has sleeper cells are already plotting to break free some 12,000 militants detained by Syrian Kurdish fighters in northeastern Syria in a "threat to local & international security."

The Kurdish fighters also control the al-Hol camp, home to more than 70,000 mostly wives and children of IS fighters.

Turkey says it wants to settle up to 2 million Syrian refugees in the zone. It currently hosts 3.6 million Syrians sheltering from Syria's more than eight-year conflict.

Turkey says the United States, which supports the Syrian Democratic Forces (SDF), a YPG-led force that defeated Islamic State fighters in Syria, is moving too slowly to set up the zone. It has repeatedly warned of launching an offensive on its own into northeast Syria, where U.S. forces are stationed alongside the SDF.

The two countries are also at odds over how far the zone should extend into Syria and who should control it. Turkey says it should be 30 km (19 miles) deep.

The ties between the allies have also been pressured over Turkey's purchase of Russian S-400 defense missiles and the trial of local U.S. consulate employees in Turkey.