A library image of US troops in Syria (Picture: US Department of Defense).
US Defense Secretary Mark Esper says President Donald Trump has ordered all US troops in northern Syria to move south.
Mr Esper said the conflict between Turkish forces and the US-backed Syrian Kurdish fighters had become "untenable" for the US military.
However, Mr Esper said they have not been ordered to leave Syria entirely.
He did not say how many US troops will leave the area but that they represent the majority of the 1,000 troops in the country.
Mr Esper also said the US believes the Kurds are about to "cut a deal" with the Syrian regime's army and Russia to enable a counter-attack against Turkey and its Syrian proxy forces.
The announcement comes after Kurdish officials said hundreds of so-called Islamic State (IS) supporters had escaped from a camp.
Earlier this week, the US withdrew troops from the north-east of the country - a decision which has faced widespread criticism.
Mr Trump said he wanted the US to stop getting involved with "endless wars".
However, he has been accused of endangering regional stability and risking the lives of Syrian-Kurdish allies.
US forces fought alongside the Kurdish-led Syrian Democratic Forces (SDF) against so-called IS - playing a key role in the collapse of the group's self-proclaimed caliphate.
Turkey, which views the SDF as a terrorist organisation, launched an offensive on Wednesday.
Ankara says it is an anti-terror operation and that it wants to create a 20-mile "safe zone" along the Syrian border.
The United Nations says more than 100,000 people in north-east Syria have had to leave their homes.
Boris Johnson and Foreign Secretary Dominic Raab have both called on Turkey to halt the assault.