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 forces to be a terrorist insurgency, and has long sought to end American support for the group. But the Kurdish fighters, which are part of the Syrian Democratic Forces, or S.D.F., have been the United States’ most reliable partner in fighting the Islamic State in a strategic corner of northern Syria.

[Related: Why Turkey is figthing the Kurds in Syria, explained.]

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 operations against the Islamic State, or ISIS, and to act as a critical counterweight to Iran and Russia.