BEIRUT  — Syrian state-run media said Thursday a rare U.S. strike on government-backed troops in eastern Syria killed and wounded dozens of tribal fighters, calling it a new effort “to support terrorism.”

The U.S.-led coalition said its action was in “self-defense,” citing a major attack on its allied forces and U.S. advisers in Deir el-Zour province by at least 500 fighters, battle tanks, howitzers and mortars.

The overnight attack, which a U.S. military official earlier said killed about 100 of the attackers, added another layer to the Syrian conflict that is once again plunging into a new and unpredictable spiral of violence.

The attack in Deir el-Zour province in northeastern Syria occurred in crowded battle space. A stronghold of Islamic State militants until late last year, the province also was the group’s main source of oil revenue. U.S.-backed Kurdish-led forces have been fighting for control of Deir el-Zour with rival Russian-backed Syrian troops that are reinforced by Iranian-supported militias.

The U.S.-backed forces control areas east of the Euphrates River and most of the oil and gas fields, while government forces are based in the west. Previous friction was reported before, but Wednesday’s incident appeared to be the largest since last year. The IS militants have been dispersed in the province, which also has a number of U.S. bases.