WASHINGTON—The U.S. military is crafting plans to keep nearly 1,000 forces in Syria, U.S. officials said, a shift that comes three months after President Trump ordered a complete withdrawal and is far more than the White House originally intended.

Protracted administration talks with Turkey, European allies and U.S.-backed Kurdish fighters have so far failed to secure an agreement to create a safe zone in northeastern Syria, part of Mr. Trump’s plan for leaving Syria.

The...