"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 towns are in ISIS-held areas east of the Euphrates River near Al Boukamal in Deir ez-Zor Govenorate, the monitor added.
The Observatory later said that at least 54 people were killed or injured, and the strikes may have come from Iraqi or coalition forces.
A spokesperson for the anti-ISIS coalition said "the Coalition or our partner forces may have conducted strikes in the vicinity" on Thursday.
"We are forwarding the report to our Civilian Casualty Cell for further assessment on this allegation," the spokesperson for the coalition US Col. Sean Ryan told Rudaw English.
The US-led anti-ISIS coalition, partnered forces, and the Iraqi military have conducted air strikes on the eastern side of the Euphrates, in-line with a Russian/Coalition demarcation line that runs roughly along the Euphrates.
In the first week of July, the coalition reported several anti-ISIS strikes in the areas of Al Boukamal and across the border with Iraq in al-Qaim.
The Syrian Democratic Forces and the Iraqi military did not not immediately comment.
The Syrian Network for Human Rights reported earlier in July that there have been 186 massacres perpetrated by different parties across the country so far in 2018.
Last updated at 3:05 p.m.
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