BEIRUT, Lebanon — The American-backed coalition in Syria said Thursday that it had repelled an attack by forces supporting the Assad government, carrying out deadly strikes in a rare confrontation between competing factions that have both fought the Islamic State.
The clash, on Wednesday night, occurred in an area of eastern Syria where government troops and their allies have competed with the Kurdish-led Syrian Democratic Forces, allied with the United States, to seize territory rich in oil and natural gas.
There were unconfirmed and conflicting reports about the number of casualties from the airstrikes.
The fighting offered a glaring example of the new risks posed by a host of combatants in the region, including Iran, Russia, Syria, Turkey and the United States, who, though wary of one another, had a shared interest in defeating the Islamic State. Now that the militant group has been driven from most of its territory, the conflicting interests are leading to increasing friction and unpredictable escalations, like Turkey’s recent assault on a Kurdish-controlled region.
The United States military’s Central Command said on Thursday that “Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces.” It said that at the time of the clash, international coalition troops were with the S.D.F., a mix of Kurdish, Arab and other fighters, but it did not say whether any were American.
“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” the Islamic State. It said they had left “scores of persons dead and others injured.”
A Syrian military officer, reached via online chat, said that at least 100 pro-government fighters had been killed. He said they were members of a militia loyal to President Bashar al-Assad, called the Bakr group, part of a small Shiite population in southeast Syria’s Deir al-Zour Province.
Syrian state media said scores had been killed, but other news accounts put the death toll far lower. A pro-government commander quoted by Reuters said seven had been killed and 27 wounded.
An American official said the precise identity of the pro-government militia force was unclear but that it had been massing for about a week. The official, who spoke on condition of anonymity, also said the Americans had stayed in constant communication with Russian military counterparts, who had made no effort to stop the airstrikes.
Middle East experts said that direct fighting between Americans and Russians would represent a dangerous escalation in the Syrian fighting.
Kremlin officials said that no Russians had been involved. In a statement, Russia’s Defense Ministry said that the coalition strike had been carried out by helicopters.
The Russian Defense Ministry also appeared to partly blame the pro-government militia members for the episode, saying they had not coordinated their movements with the Russians ahead of time. But the ministry also said it showed that “the true aim of the illegal presence of American forces on Syrian territory is not the fight against the Islamic State,” but the “capture and retention of economic assets.”
At the United Nations, Ambassador Vassily A. Nebenzia of Russia said he intended to raise the American airstrikes during closed consultations of the Security Council. But there was little or no expectation that the council, long divided over the Syria conflict, would issue a statement on the airstrikes.
Speaking to reporters, Mr. Nebenzia rejected the American explanation that the airstrikes had been carried out for defensive reasons.
“We were invited by the government of Syria to fight terrorists since 2015,” he said. “The U.S. was never invited to Syria. So who is the aggressor, tell me?”
Aaron David Miller, a Middle East analyst at the Wilson Center, said the forces backing the Syrian government seemed to be testing the coalition’s resolve. But, he added, it was not clear whether the clash marked a new phase of the war in which the United States and its allies would enter more direct confrontation with forces loyal to Mr. Assad.
“We’re now protecting a territory the size of Indiana and deepening our commitment to the S.D.F., with no sense of where this is going, no sense of strategy, no sense of endgame,” said Mr. Miller, who was a State Department official in Republican and Democratic administrations.
The fighting in Deir al-Zour Province happened between the Euphrates River and the Iraqi border.
The United States Central Command said the pro-Assad forces had attacked about five miles east of “the agreed-upon Euphrates River de-confliction line.”
The area has developed into a potential flash point, with the S.D.F. and international coalition holding most of the territory northeast of the river — including a significant portion of Syria’s oil production — and the pro-Assad alliance holding most of the land to the southwest.
Tensions heightened recently when the United States announced that it would help to stabilize the S.D.F.-controlled areas, ensure that the Islamic State could not make a comeback, and deter expanding Iranian influence. That angered enemies and allies alike, with the Turks accusing the United States of cementing a Kurdish autonomous zone — as well as de facto Kurdish control over adjacent majority-Arab areas.
Turkey then invaded Afrin, an area farther west that is controlled by another Kurdish group, the Y.P.G., which Ankara considers a terrorist threat. The United States did not intervene, but Turkey has threatened to attack S.D.F. forces in the eastern town of Manbij, where American advisers are present.
American military officials warned on Wednesday that they would defend the area, creating a tense standoff with a NATO ally.
Elsewhere in Syria on Thursday, the government continued attacks on two of the main remaining rebel-held areas that escalated over the past month and have reached a crescendo in recent days.
At least 60 people were killed in the besieged, rebel-held Damascus suburbs of Eastern Ghouta, according to rescue workers known as the White Helmets. The toll has reached more than 400 since late December, among them more than 100 children.
Heavy bombardment also continued in the northern rebel-held province of Idlib, where at least 12 people died, adding to scores killed in recent days.
In the capital, Damascus, rebel shelling hit the government-controlled Old City for a third consecutive day, apparently in retaliation for the Eastern Ghouta attacks. At least 13 people have been killed in recent shellings, according to state media, shattering what had been a relative calm in recent months.