The American-backed coalition in Syria has repelled an attack by forces supporting the Assad government, carrying out deadly strikes Wednesday night, in a rare confrontation between competing factions that have both fought the Islamic State.

“Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces,” the United States military’s Central Command said in a statement. The S.D.F., a mix of Kurdish, Arab and other fighters, is allied with the international coalition, including the United States, and Central Command said that coalition troops were with the S.D.F. forces at the time of the attack.

“In defense of coalition and partner forces, the coalition conducted strikes,” it said.

The Syrian state news agency, Sana, described the strikes as aggression by the coalition against “popular forces that were fighting Daesh,” a term for the Islamic State. It said they had left “scores of persons dead and others injured.”

It was not clear whether those killed were the forces of the Syrian president, Bashar al-Assad, or of his allies.

The clash occurred in Deir al-Zour Province in eastern Syria, between the Euphrates and the Iraqi border. Syrian government forces in the area and their allies, including Russia, have competed with the S.D.F. to seize territory rich in oil and natural gas that had been controlled by retreating Islamic State militants. Central Command said the pro-government forces had attacked about five miles past “the agreed-upon Euphrates River de-confliction line.”

Aaron David Miller, a Middle East analyst at the Wilson Center, said the crucial question was whether the pro-government advance and the sharp response were isolated instances, or the beginning of “a new phase” in the Syrian war. Such a shift would put the United States and its allies in more direct confrontation not only with Mr. Assad, but also with Russia. Also among Mr. Assad’s allies are Iranian-backed militias, including the Lebanon-based militant group Hezbollah.

Farther west in Syria, Americans are also in a standoff with Turkey, a NATO ally, which is fighting Kurdish-led forces.