Donald Trump on Sunday gave his blessing to a Turkish operation targeting Kurdish militants in northern Syria, saying the United States will stand aside as Ankara launches its "long-planned" operation.

The move marks a major shift in US policy, and effectively abandons an American ally in the battle against the so-called Islamic State group that took over swathes of Syria.

"Turkey will soon be moving forward with its long-planned operation into Northern Syria," a White House statement issued late Sunday evening 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," it said, using another acronym for the Islamic State (IS) group.

The statement, which followed a call between the US president and his Turkish counterpart Recep Tayyip Erdogan, also criticized "France, Germany, and other European nations" for what it said was their refusal to take back citizens detained in northern Syria.

"Turkey will now be responsible for all ISIS fighters in the area captured over the past two years in the wake of the defeat of the territorial 'Caliphate' by the United States," the statement said.

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

US troops in Syria
The White House has said the US will not support a cross-border offensive in northern Syria by Turkey Photo: AFP / Delil SOULEIMAN Delil Souleiman/AFP

Erdogan also expressed his "frustration over the US military and security bureaucracy's failure" to implement an August deal establishing a buffer zone on the Turkish border.

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

Kurdish authorities in northeast Syria have denounced Erdogan's comments and urged the international community to step in to stop a Turkish assault on Syrian territory under their control.

Ankara wants to push the Syrian Kurdish People's Protection Units (YPG) militia from its border, saying that the group is a "terrorist" offshoot of Kurdish insurgents in Turkey.

Ankara wants to urgently establish a "safe zone" for some of the 3.6 million refugees who fled the war in Syria, amid growing public backlash against their presence in Turkey, and a clamour for them to be returned.

The Kurdish-led Syrian Democratic Forces (SDF) warned that any Turkish attack would reverse the gains made against IS and make Syria a zone of permanent conflict.

"Erdogan's threats are aimed to change the security mechanism into a mechanism of death, displace our people & change the stable & secure region into a zone of conflict and permanent war," the SDF said in a series of tweets.

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