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.