The United States is poised to evacuate about 1,000 US troops from northern Syria amid an ongoing Turkish offensive into the region, Defense Secretary Mark Esper said on Sunday, calling the situation “untenable” for US forces.

The withdrawal of the troops from the region comes after US President Donald Trump abruptly shifted policy and withdrew some US troops deployed to support Kurdish forces in the fight against the militant Islamic State group.

That decision in turn helped open the door for Turkey to launch an offensive against the Syrian Democratic Forces (SDF).

“In the last 24 hours, we learned that [the Turks] likely intend to extend their attack further south than originally planned, and to the west,” Esper said in a pre-taped interview with “Face the Nation” on CBS.

“We also have learned in the last 24 hours that the ... SDF are looking to cut a deal, if you will, with the Syrians and the Russians to counterattack against the Turks in the north.” Esper said he spoke with Trump on Saturday night, and that the president directed the US military to “begin a deliberate withdrawal of forces from northern Syria.”

Meanwhile, a small number of US troops have left an outpost in the town of Ain Issa in northern Syria because of concerns about a Turkish incursion that is pushing into Syria, two US defense official told Reuters on Sunday.

The officials, speaking on condition of anonymity, said the troops had moved because of concerns they could be caught in the middle of the Turkish offensive.

Turkey's offensive has raised alarm bells around the world amid concerns about its impact on civilians and the possibility of Islamic State militants escaping from Kurdish-led authorities.

On “Fox News Sunday,” Esper said the United States knew ahead of time about Turkish President Recep Tayyip Erdogan's intentions to cross the border into Syria.

“It was clear to me that President Erdogan was committed to coming in. He informed us that he was coming in. He didn't ask permission.” He added that the United States simply did not have enough troops to stop a Turkish advance and they needed to be taken out of harm's way.

“There is no way they could stop 15,000 Turks from proceeding south,” he said.