WASHINGTON — In a major shift in United States military policy in Syria, the White House said on Sunday that President Trump had given his endorsement for a Turkish military operation that would sweep away American-backed Kurdish forces near the border in Syria.
Turkey considers the Kurdish fighters to be a terrorist insurgency, and has long sought to end American support for the group. But the Kurdish group, known as the Syrian Democratic Forces, or S.D.F., has been the United States’ most reliable partner in fighting the Islamic State in a strategic corner of northern Syria.
Now, Mr. Trump’s decision goes against the recommendations of top officials in the Pentagon and the State Department who have sought to keep a small troop presence in northeast Syria to continue counterinsurgency operations against the Islamic State, or ISIS, and to act as a critical counterweight to Iran and Russia.
Administration officials said that Mr. Trump spoke directly with President Recep Tayyip Erdogan of Turkey on the issue on Sunday. And the officials indicated that the 100 to 150 United States military personnel deployed to that area would be pulled back in advance of any Turkish operation but that they would not be completely withdrawn from Syria.
“Turkey will soon be moving forward with its long-planned operation into Northern Syria,” the White House said in a statement released just before 11 p.m. in Washington. “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.”
It was unclear how extensive the Turkish operation would be, or whether Turkish forces would clash with the American-backed Kurds, a development that could jeopardize many of the counterterrorism gains achieved by the American military in the fight against ISIS.
Soner Cagaptay, the director of the Turkish Research Program at the Washington Institute for Near East Policy, and author of “Erdogan’s Empire: Turkey and the Politics of the Middle East,” said in a telephone interview that a Turkish incursion uncontested by the United States would allow Turkey to cut another swath into Kurdish-controlled territory in Syria. That would give Mr. Erdogan a ready place to send hundreds of thousands of Syrian refugees and prove yet again his influence with Mr. Trump on Syria policy.
“It’s a quite a significant development,” Mr. Cagaptay said.
Last December, Mr. Trump called for a complete United States withdrawal from Syria, but ultimately reversed himself after a backlash from Pentagon, diplomatic and intelligence officials, as well as important allies in Europe and the Middle East.
Mr. Erdogan has demanded a “safe zone” for his nation to run 20 miles deep and 300 miles along the Turkish-Syrian border east of the Euphrates. That area, he has said, would be reserved for the involuntary return of at least a million Syrian refugees now inside Turkey. Mr. Erdogan has threatened to send a wave of Syrian migrants to Europe instead if the international community does not support the initiative to send them back to Syria.
Since early August, the American and Turkish militaries have been working together on a series of confidence-building measures — including joint reconnaissance flights and ground patrols — in a 75-mile-long strip of that 300-mile border area.
American-backed Kurdish forces have pulled back several miles and destroyed fortifications in that area.
The pace of these operations has not been fast enough for Mr. Erdogan, and last week he began indicating he planned to launch an incursion across the border. He did the same thing over the summer, prompting a flurry of American diplomatic activity bolstered by the military confidence-building measures.
Defense Secretary Mark T. Esper and Gen. Mark Milley, the new chairman of the Joint Chiefs of Staff, both called their Turkish counterparts last week to try and reduce tensions. But unresolved threats from Turkey apparently resulted in the decision by Mr. Trump on Sunday.
American officials contacted late Sunday would not say how far back from the Turkish border American troops would redeploy, or whether this signals the beginning of a larger overall withdrawal of the 1,000 American troops now in northeast Syria conducting and supporting counterterrorism operations.
One official, who spoke on condition of anonymity to describe a fluid military situation, said that American forces were pulling back from northeast Syria to “get out of the way.”
Officials described a military and political tension as the American military is pulled between two important allies in the civil war in Syria. Turkey is a major NATO ally, but the Kurdish S.D.F. forces have been a partner in the fight against ISIS.
“We are not going to support the Turks and we are not going to support the S.D.F.,” the official said. “If they go to combat, we’re going to stay out of it.”
The White House statement and its ramifications come as the Islamic State is gathering new strength, conducting guerrilla attacks across Iraq and Syria, retooling its financial networks and targeting new recruits at an allied-run tent camp, American military, counterterrorism and intelligence officers say.
Though Mr. Trump hailed a total defeat of the Islamic State this year — and asserted its territorial demise in Sunday night’s statement — defense officials in the region see things differently, acknowledging that what remains of the terrorist group is here to stay.
A recent inspector general’s report warned that a drawdown ordered by Mr. Trump this year — from 2,000 American forces in Syria to less than half of that — has meant that the American military has had to cut back support for Syrian partner forces fighting ISIS. For now, American and international forces can only try to ensure that ISIS remains contained and away from urban areas, officials say.
Although there is little concern that the Islamic State will reclaim its former physical territory, a self-declared Islamic caliphate that was once the size of Britain and controlled the lives of up to 12 million people, the terrorist group has still mobilized as many as 18,000 remaining fighters in Iraq and Syria. These sleeper cells and strike teams have carried out sniper attacks, ambushes, kidnappings and assassinations against security forces and community leaders.
Over the past several months, ISIS has made inroads into a sprawling tent camp in northeast Syria, and there is no ready plan to deal with the 70,000 people there, including thousands of family members of ISIS fighters.
American intelligence officials say the Al Hol camp, managed by Syrian Kurdish allies with little aid or security, is evolving into a hotbed of ISIS ideology and a huge breeding ground for future terrorists. The American-backed Syrian Kurdish force also holds more than 10,000 ISIS fighters, including 2,000 foreigners, in separate makeshift prisons.
The custody of all these people could be in jeopardy, American officials said Sunday night, depending on whether any Turkish incursion sets off a much larger conflict in northeast Syria.