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WASHINGTON — The United States appears to be heading toward a full military withdrawal from Syria amid growing chaos, cries of betrayal and signs that Turkey’s invasion could fuel a broader war.

Defense Secretary Mark Esper said Sunday that President Donald Trump had directed U.S. troops in northern Syria to begin pulling out “as safely and quickly as possible.” He did not say Trump ordered troops to leave Syria, but that seemed like the next step in a combat zone growing more unstable by the hour.

Esper, interviewed on two TV news shows, said the administration was considering its options.

“We have American forces likely caught between two opposing advancing armies and it’s a very untenable situation,” Esper said.

This seemed likely to herald the end of a five-year effort to partner with Syrian Kurdish and Arab fighters to ensure a lasting defeat of the Islamic State group. Hundreds of IS supporters escaped a holding camp amid clashes between invading Turkish-led forces and Kurdish fighters, and analysts said an IS resurgence seemed more likely, just months after Trump declared the extremists defeated.

The U.S. has had about 1,000 troops in northeastern Syria allied with the Kurdish-led Syrian Democratic Forces to combat IS. The Pentagon previously had pulled about 30 of these troops from the Turkish attack zone along the border. With an escalation of violence, a widening of the Turkish incursion and the prospect of a deepening conflict, all U.S. forces along the border will now follow that move. It was unclear where they would go.

The Pentagon chief did not say U.S. troops are leaving Syria entirely. The only other U.S. presence in Syria is at Tanf garrison, near Syria’s eastern border with Jordan. The U.S. and coalition troops there are not involved in the Kurd mission, and so it seems highly unlikely the 1,000 being moved from the north would go to Tanf.

Critics say the U.S. has betrayed the Kurds by pulling back in the face of Turkey’s invasion, but Esper said the administration was left with little choice once President Recep Tayyip Erdogan told Trump a week ago that he was going ahead with a military offensive. Esper said the Kurds have been good partners, “but at the same time, we didn’t sign up to fight the Turks on their behalf.”

The Kurds then turned to the Syrian government and Russia for military assistance, further complicating the battlefield.

The prospect of enhancing the Syrian government’s position on the battlefield and inviting Russia to get more directly involved is seen by Trump’s critics as a major mistake. But he tweeted that it shouldn’t matter.

“Others may want to come in and fight for one side or the other,” he wrote. “Let them!”