US troops have begun withdrawing from positions in northern Syria, paving the way for a Turkish operation against Kurdish fighters in the border area.
Kurdish-led forces have until now been a key US ally in Syria, where they helped defeat the Islamic State group, but Turkey regards them as terrorists.
The main Kurdish-led group called the surprise US move a “stab in the back”.
But President Donald Trump defended the pullout, saying it was time “to get out of these ridiculous Endless Wars”.
The withdrawal represents a significant shift in US foreign policy and goes against the advice of senior officials in the Pentagon and state department.
It follows a White House statement issued late on Sunday, saying US troops were stepping aside for an imminent Turkish operation.
Turkish President Recep Tayyip Erdogan said Turkey’s aim was to combat Kurdish fighters in the border area and to set up a “safe zone” for Syrian refugees currently living in Turkey.
The UN humanitarian chief in Syria, Panos Moumtzis, says aid workers are “preparing for the worst” if fighting breaks out in north-eastern Syria.
“From experience, this could result in a displacement of people, we want to make sure that we are ready,” he told reporters.
What did the White House say?
“Turkey will soon be moving forward with its long-planned operation into northern Syria,” the 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 also said Turkey would take over all responsibility for IS fighters captured by Kurdish forces over the past two years.
More than 12,000 men are in detention on suspicion of being IS members in Kurdish-controlled camps located south of Turkey’s planned “safe zone”. At least 4,000 of them are foreign nationals.
US allies betrayed
This decision risks a recasting of alliances in Syria. The Kurds may be forced to seek an accommodation with the Syrian government. The potential chaos could facilitate a resurgence of IS. Indeed, the US pullback of its forces from the border area may herald the full withdrawal of troops from Syria that Mr Trump has long wanted.
It marks a betrayal of Washington’s Kurdish allies, a betrayal that many other countries in the region will note with alarm.
Both the Saudis and the Israelis are coming to realise that Mr Trump’s robust rhetoric is rarely matched by actions.
Last month the Syria Study Group, a bipartisan body commissioned by Congress, stated in its final report that the US still has significant security interests in Syria and retains some policy levers with which to influence events there. But that is clearly not Present Trump’s view.
What has the reaction been?
A spokesman for the Kurdish-led Syrian Democratic Forces (SDF) – which occupy former IS territory in north-eastern Syria – strongly condemned the move, saying they were “assurances from the US that it would not allow any Turkish military operations against the region.”
“The [US] statement was a surprise and we can say that it is a stab in the back for the SDF,” Kino Gabriel told Arabic TV station al-Hadath.
US Republican Senator Lindsey Graham, a close ally of the president, said the withdrawal was a “disaster in the making”, adding that a pullout would be a “stain on America’s honor for abandoning the Kurds”. Mr Graham said he would introduce a Senate resolution opposing the decision and calling for it to be reversed.
Brett McGurk, former US special presidential envoy for the coalition against IS, said the announcement demonstrated a “complete lack of understanding of anything happening on the ground”.
Meanwhile, Kurdish TV in northern Iraq said the SDF had put some of its units on alert because the Turkish army had mobilised troops on the border on Monday.
What is Turkey planning?
Late on Sunday, Mr Erdogan’s office said he and President Trump had spoken on the phone about Turkey’s plan to set up a “safe zone” in north-eastern Syria.
It said the 20-mile (32km) zone along the border was needed to combat “terrorists” and create “the conditions necessary for the return of Syrian refugees”.
Turkey considers the Kurdish YPG militia – the dominant force in the SDF alliance – an extension of the banned Kurdistan Workers’ Party (PKK), which has fought for Kurdish autonomy in Turkey for three decades.
Turkey hosts more than 3.6 million Syrians who fled the civil war that began in 2011. It wants to move up to two million of them into the zone.
In his call with Mr Trump, President Erdogan also expressed his “frustration over the US military and security bureaucracy’s failure” to implement an agreement reached in August about the zone, his office said.
How large will the Turkish operation be?
Analysis by Quentin Sommerville, Middle East correspondent
Invasion or incursion? That’s the uppermost question about Turkish action in north-eastern Syria. Initial indications would point to a limited incursion by Turkey along a 60-mile (100km) stretch between the towns of Tal Abyad and Ras al-Ain.
It’s a sparsely populated, mostly Arab area. American forces have already withdrawn from four border positions there, but they haven’t withdrawn from any positions further east and west.
Similarly, the big Kurdish towns and cities along the border – Kobane, Qamishili and others – remain calm. There’s been no call for people to evacuate.
Prisons full of IS foreign fighters are further south and will remain under Kurdish control – if Turkey restricts itself to a limited assault.
However, it may not stop there: regime sources are calling it “a full incremental invasion”. So British and American special forces have for months been making preparations for a partial or full withdrawal from the area if the situation escalates.