An airstrike has killed 28 civilians in one of the last holdouts of the Daesh terror group in Syria's eastern province of Deir el-Zour, a war monitor said Friday.

It was not immediately clear if the raid that took place late on Thursday on a "gathering of civilians" near the village on Al-Soussa near the Iraqi border was carried out by an Iraqi plane or one of the U.S.-led coalition fighting Daesh, the Syrian Observatory for Human Rights said.

Iraqi warplanes have recently carried out strikes in eastern Syria, while coalition aircraft have been supporting the PKK terror group's Syrian wing, the People's Protection Units (YPG).

State news agency SANA reported the strike late Thursday, saying more than 30 civilians were killed and accused the U.S.-led coalition.

The coalition was not immediately available for comment.

Daesh terrorists swept across Syria and Iraq in 2014, declaring autonomy in areas they controlled.

They have since lost most of that territory to various offensives, but still retain pockets in Syria including in Deir El-Zour.

Al-Soussa lies to the east of the Euphrates River, which runs diagonally through Deir El-Zour, splitting the province in half.