U.S.-Backed Coalition in Syria Strikes Pro-Assad Forces

BEIRUT — The U.S.-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.

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 U.S. military’s Central Command said 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 SDF, 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.”

It was not clear whether the casualties were the forces of Syrian President Bashar Assad or of his allies, which include Russia and Iran-backed militias such as the Lebanon-based militant group Hezbollah. 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 was carried out by helicopters, and that it had wounded 25 Syrian insurgents.

The Defense Ministry charged that the U.S. aim in Syria was “capture and retention of economic assets.” The Foreign Ministry said that the U.S. presence was giving shelter to Islamic State fighters.

At the United Nations, Ambassador Vassily A. Nebenzia of Russia said he intended to raise the U.S. 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, Nebenzia rejected the U.S. 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 Assad.

“We’re now protecting a territory the size of Indiana and deepening our commitment to the SDF, with no sense of where this is going, no sense of strategy, no sense of endgame,” said Miller, who was a State Department official in Republican and Democratic administrations.

The fighting took place Deir el-Zour province in eastern Syria, between the Euphrates and the Iraqi border. Central Command said the pro-Assad forces had attacked about 5 miles east of “the agreed-upon Euphrates River deconfliction line.” The area has developed into a potential flash point, with the SDF 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 SDF-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 YPG, which Ankara considers a terrorist threat. The United States did not intervene, but Turkey has threatened to attack SDF forces in the eastern town of Manbij, where U.S. advisers are present.

U.S. military officials warned Wednesday that they would defend the area, creating a tense standoff with a NATO ally.