The US will step aside for an imminent Turkish operation against Kurdish-led forces in north-eastern Syria, the White House has said, in a major shift.
Kurdish militias were key to defeating Islamic State (IS) militants in Syria, but Turkey regards them as terrorists.
The US - which has hundreds of troops in the border area - has reportedly begun to withdraw them.
Kurdish YPG fighters have until now have received significant support from the US and have condemned the pullout.
In January, President Trump threatened to "devastate Turkey economically" if it attacked Kurdish forces.
However, the White House statement issued on Sunday makes no reference to the YPG.
The statement followed a phone call between President Donald Trump and Turkish President Recep Tayyip Erdogan.
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 that Turkey would take over all responsibility for IS fighters captured by Kurdish forces over the past two years.
"The United States government has pressed France, Germany, and other European nations, from which many captured ISIS fighters came, to take them back but they did not want them and refused.
"The United States will not hold them for what could be many years and great cost to the United States taxpayer."
What is Turkey's plan?
Late on Sunday, Mr Erdogan's office said that 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 move was needed to combat "terrorists" and create "the conditions necessary for the return of Syrian refugees to their native country".
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 the refugees from its territory 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 about the zone reached in August, his office said.
On Saturday, he warned that Turkish forces could launch a cross-border offensive in the coming days.
He has not given any details about the scale of the planned offensive.
How have the Kurds reacted?
On Monday the Kurdish-led Syrian Democratic Forces (SDF) - who occupy former IS territory in the north east of Syria - said US forces had begun to withdraw from border areas.
The statement added that the move would have a "great negative" impact on the group's war against IS.
The Syrian Observatory for Human Rights, a UK-based monitoring group, also said US forces had pulled back from key positions.
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's the background?
The YPG was a major part of the Syrian Democratic Forces, the US-supported force that defeated IS in Syria.
Turkey considers the YPG an extension of the banned Kurdistan Workers' Party (PKK) which has fought for Kurdish autonomy in Turkey for three decades.
The YPG denies any direct organisational links to the PKK.
Turkey has previously condemned the US for supporting the YPG.