The U.S. “would certainly respond as necessary” to protect local partners, Pahon said. But in the event of an Assad advance on Raqqa, “I don’t think our response would necessarily be to move in a whole bunch of tanks,” as diplomatic options would be pursued, he said.

The Kurds face another threat from Turkey, which also has troops inside Syria and strongly opposes any move toward Kurdish autonomy there. Last month, after Turkish planes bombed Kurdish fighters, the U.S. had to send soldiers to shield one of its allies from another.

Farther south, it’s the forces of the Assad-Russia-Iran alliance — which has shown little sign of internal dissension — that are leading the fight to oust Islamic State. They’re pushing toward Deir Ezzor, a city about 140 kilometers (85 miles) southeast of Raqqa, which has been besieged by the jihadists since 2015, and also offers control of the frontier with Iraq.

Russia attacked Islamic State targets including heavy weaponry and fighters near the ancient city of Palmyra — southwest of Raqqa and Deir Ezzor — with four cruise missiles fired from a warship and submarine, the Defense Ministry in Moscow said on Wednesday.