"Tens of civilians" were gathering at an ice factory between al-Sussa and al-Baghuz Foqani on Thursday, the Syrian Observatory for Human Rights reported, naming the towns of Hajin, al-Sousse, and al-Shafa. The group said it was unclear who had carried out the strike near the village of al-Soussa, close to the Iraqi border.
Daesh fighters swept across Syria and Iraq in 2014, declaring a cross-border "caliphate" in areas they controlled.
SANA, the official news agency of the Syrian government, reported the strike claimed 30 civilians were killed and blamed the US-led coalition of conducting the attack.
Iraqi warplanes have recently carried out strikes against ISIS in eastern Syria, while coalition aircraft have been supporting Kurdish-led fighters battling the extremists.
The US did not contest this assessment, with a spokesman saying that US coalition forces "may have conducted strikes in the vicinity" and promised that there would be an assessment at some point regarding civilian casualties.
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They will be set free in the United States pending the outcome of their immigration cases, which can take several years. A different judge, in Los Angeles, on Monday dismissed as "dubious" and "unconvincing" the U.S.
The US-led coalition could not immediately be reached for comment by phone and did not immediately respond to an emailed request for comment.
The U.S. -led coalition is fighting the remnants of Islamic State on both sides of the border between Syria and Iraq.
At least 35 civilians were killed late Thursday by airstrikes on a Daesh-held position in Syria's Deir ez-Zor province.
Pro-government forces regained Deir Ezzor city, as well as nearby oil and gas fields, from IS last November as part of a campaign that has seen it assert its control over much of the east of the country.