WASHINGTON—The U.S. wants to assemble a coalition of Western nations to create and potentially enforce a new buffer zone in northern Syria, U.S. officials said, but none have yet agreed to the proposal, which includes a promise of American military assistance.

The difficulty in getting allies on board with the plan, details of which hadn’t previously been disclosed, is the latest challenge administration officials face as they search for a way to satisfy President Trump’s withdrawal order and avoid potentially adverse effects of a pullout, including an Islamic State rebound or a war between Turkish forces and Kurdish fighters.

The administration hopes to persuade allies including the U.K., France and Australia to take responsibility for northern Syria, both to address Turkish concerns about Kurdish separatists in Syria while keeping Turkey’s forces away from U.S.-backed Syrian Kurdish fighters who have been battling Islamic State.

Turkey, which has vowed to create its own safe zone, has objected to the idea.

Mr. Trump, citing success against Islamic State in Syria, ordered the more-than-2,000 American forces out of Syria in December, a move Republicans and Democrats criticized as premature. The order means troops will redeploy from Syria over the next four to six months, officials said.