Mr. Trump has since reversed his withdrawal order to keep under 1,000 American troops in Syria. But given the uncertainty of their mission — and how long they will remain — allies are hedging their support for the continued fight.

European efforts to provide humanitarian aid, resettle refugees and clear explosives have largely been put on hold until the United States can assure that the area is safe. In a statement on Wednesday, the French foreign ministry noted that Syrian territory was wrested from the Islamic State only seven months ago, and urged against “any unilateral initiative that could undermine this achievement.”

One foreign diplomat said Thursday’s meeting would focus on what he described as a loss of clear strategy by the United States in Syria. Another predicted the high-level talks would amount to little more than a meet-and-greet. Both spoke on condition of anonymity to discuss the sensitive diplomacy.

“It’s quite clear that the president has been convinced to retain troops on the only basis that might have been of interest to him — the existence of oil,” said Charles R. Lister, a senior fellow at the Middle East Institute. “But the much bigger U.S. government apparatus is trying to use that as a cover to form a more meaningful, less ambiguous and more sustainable strategy focused on countering terrorism while reasserting leverage over Damascus.”

How to deal with an estimated 10,000 Islamic State fighters who are detained in Kurdish-controlled camps in northeast Syria will be among the top topics at Thursday’s talks.

The vast majority of the fighters — about 8,000 — are Syrian and Iraqi. They are too dangerous to release, yet too vulnerable to abuse by security forces if they were to be transferred to the custody of their respective governments for trial, officials said.