Istanbul: The United States said it was pulling troops from northeast Syria, in a major policy shift which opens the way for a Turkish offensive against Kurdish-led forces and hands Turkey responsibility for thousands of Islamic State captives.

The move angered the Kurdish-led forces who have been Washington's most capable partner in fighting Islamic State in Syria. The forces, known as the Syrian Democratic Forces (SDF), accused Washington of reneging on an ally, warning it would have a "great negative" impact on the war against the jihadists.

Reflecting concern about potential harm to civilians, a UN official said civilians must be spared in any Turkish military operation in northeast Syria, adding the United Nations hoped that displacement and atrocities can be prevented.

"We are hoping for the best but preparing for the worst," Panos Moumtzis, U.N. regional humanitarian coordinator for the Syria crisis, told reporters in Geneva.

A US official said American troops had withdrawn from two observation posts on the border, at Tel Abyad and Ras al-Ain, and had told the commander of the SDF that the United States would not defend the SDF from an imminent Turkish offensive.

"Turkey will soon be moving forward with its long-planned operation into Northern Syria," the White House said after President Donald Trump spoke to Turkey's President Tayyip Erdogan on Sunday.

"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 in a statement.

Turkey has long argued for the establishment of a 20-mile (32 km) "safe zone" along the border, under Turkish control, driving back the Syrian Kurdish YPG militia - which is the dominant force in the SDF alliance and which Ankara considers a terrorist organisation and a threat to its national security.

The United States helped the YPG defeat Islamic State militants in Syria, and had been seeking a joint 'security mechanism' with Turkey along the border to meet Turkey's security needs without threatening the SDF.

The SDF accused Washington of turning its back on an ally.

"The American forces did not fulfill 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," it said in a statement.

SDF official Mustafa Bali said US forces were "leaving leaving the areas to turn into a war zone".

Islamic State captives

The White House statement appeared to hand over to Turkey responsibility for captured Islamic State jihadists who are currently held in SDF facilities to the south of Turkey's initially proposed safe zone.

"Turkey will now be responsible for all ISIS fighters in the area captured over the past two years," it said.

The statement also made pointed reference to Washington's European allies, saying many of the captured IS fighters came from those countries, which had resisted US calls to take them back.

"The United States will not hold them for what could be many years and great cost to the United States taxpayer," the White House said.

In the first Turkish comment following the statement, Erdogan's spokesman Ibrahim Kalin said Turkey's "safe zone" plan was within the framework of Syria's territorial integrity.

"The safe zone has two aims: to secure our borders by clearing away terrorist elements and to achieve the return of refugees in a safe way," Kalin wrote on Twitter.

"Turkey is powerful and determined," he added.

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

After the statements, Turkey's lira was trading weaker at 5.7150 against the US dollar, compared with a close of 5.70 on Friday. After the Erdogan-Trump phone call, the Turkish presidency said the two leaders had agreed to meet in Washington next month. It said that during the call Erdogan had expressed his frustration with the failure of US military and security officials to implement the agreement between the two countries.

The NATO allies agreed in August to establish a zone in northeast Syria along the Turkish border. Turkey says the United States moved too slowly to set up the zone. It has repeatedly warned of launching an offensive on its own on northeast Syria.

Ties between the allies have also been pressured over Turkey's purchase of Russian S-400 defence missiles and the trial of local US consulate employees in Turkey.