An explosion in eastern Syria on Wednesday killed at least three people, according to reports. A war monitoring group said the blast was likely caused by a drone strike that targeted Iran-backed militiamen.

No group claimed responsibility for an attack in the area, and reports about what had happened were sketchy. A Britain-based opposition war monitor, the Syrian Observatory for Human Rights, said four people died when the strike hit a building housing Iran-backed militiamen in the province of Deir el-Zour. It said eight people were wounded.

A local activist collective, Deir Ezzor 24, reported that the building was used as base for Iran’s paramilitary Revolutionary Guard, a key ally of Syrian President Bashar al-Assad.

Syrian state media, however, said a mine “left by terrorists” — a term often used by the government for opposition forces battling Assad’s side in the war — detonated in the area, killing three people and wounding seven. The state media reports showed photographs of a building with several collapsed floors, as well as a destroyed truck.

Syria’s 12-year civil war has killed at least 300,000 people and displaced half the country’s population of 23 million.

The militant Islamic State group once had a stronghold in much of northeastern Syria as part of its so-called caliphate. Today, several groups, including U.S.-backed Kurdish-led forces as well as Syrian government troops with their Russian and Iranian allies, control the area. However, IS sleeper cells still carry out deadly attacks there.

Past attacks on Iranian and Iranian-linked targets in eastern Syria have been attributed to either the United States or Israel, though the latter rarely acknowledges its involvement in such attacks.