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The raid late Thursday hit a "gathering of civilians" at an ice factory near the village of al-Soussa near the Iraqi border, the Syrian Observatory for Human Rights reported.

It was not immediately clear if the strike was carried out by an Iraqi plane or by one of the US-led coalition fighting the Islamic State group, the monitor said. 

It added 27 IS extremists were also killed in the attack.

Iraqi warplanes have been carrying out strikes in eastern Syria in what are some of the last pockets of IS control. Coalition aircraft have also been supporting Kurdish-led fighters battling the extremist group. 

Syria's state-run SANA news agency also reported the strike, saying more than 30 civilians were killed. It accused the US-led coalition of the air strike. 

In a written statement, the US-led alliance said: "The coalition or our partner forces may have conducted strikes in the vicinity of Al-Soussa and Baghour Fukhani" on Thursday.

"We are forwarding the report to our Civilian Casualty Cell for further assessment on this allegation," it said.

IS fighters overran much of Syria and Iraq in 2014, declaring a cross-border "caliphate" in areas they controlled. The group held as much as one-third of Iraq at its peak, including the city of Mosul.

But the militants have since lost most of that territory, in Syria seeing their presence largely reduced to the country's vast desert and a few villages near the frontier with Iraq.

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

Russia-backed regime forces have pushed back the jihadists on the western side of the Euphrates, while the US-backed and Kurdish-dominated Syrian Democratic Forces alliance have battled them to its east.

Both sides have mostly avoided running into each other and a deconfliction line exists to avoid such incidents. But there have been exceptions.

In April, rare clashes broke out between both sides, killing more than a dozen combatants. In February, the US-led coalition backing the SDF carried out air strikes in Deir az-Zour province that killed at least 100 pro-regime fighters.

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