The Pentagon said on Sunday US President Donald Trump had ordered the withdrawal of up to 1 000 troops from northern Syria - almost the entire ground force in the war-torn country - amid an intensifying Turkish assault on Kurdish forces.
Defense Secretary Mark Esper said the move came after the US learned that Turkey was pressing further into Syria than expected.
The Kurds - with whom the US partnered to combat the Islamic State (IS) group - later announced they had reached a deal with the regime in Damascus to deploy Syrian troops near the border to confront the Turkish offensive.
"We have American forces likely caught between two opposing advancing armies and it's a very untenable situation," Esper told CBS' Face the Nation.
"So I spoke with the president last night after discussions with the rest of the national security team and he directed that we begin a deliberate withdrawal of forces from northern Syria."
Esper said he was unable to provide a timeline because of the changing situation on the ground, but added that the withdrawal would be carried out in a "very safe, deliberate manner".
"It's a very terrible situation over there, a situation caused by the Turks... Despite our opposition they decided to make this incursion into Syria," Esper told CBS.
"And at this point in time in the last 24 hours we learned that they likely intend to expand their attack further south than originally planned and to the west."
Esper also pointed to a possible deal - confirmed later on Sunday by the Kurdish administration in northern Syria - on a Syrian troop deployment near the border with Turkey to help the Kurdish-led Syrian Democratic Forces (SDF) confront Ankara's offensive.
The US withdrawal announcement came after its troops came under artillery fire on Friday which the Pentagon said was from Turkish positions.
Asked if troops would be leaving the country altogether, Esper clarified that the withdrawal was just from northern Syria, "which is where most of our forces are".
Fighting raged Sunday along the border on the fifth day of an offensive that has provoked an international outcry and left dozens of civilians and fighters dead.
"The president is simply derelict in his duty as president. He is making Americans much less safe, he is undoing years of work to curb (IS)," top Senate Democrat Chuck Schumer said in a statement on Sunday.
"He is making America more susceptible to terrorism by his actions."