The Syrian Observatory for Human Rights, a Britain-based monitor, said children were among those killed as they tried to cross the river aboard rafts, escaping from areas where Russian-backed regime forces are battling the Daesh group.
Two campaigns are being fought against the jihadist group in east Deir Ezzor, with one on the western side of the Euphrates river that slices diagonally across the province led by Syrian troops and backed by ally Russia.
The second is being fought by the US-backed Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, on the eastern bank of the river.
The Observatory relies on a network of sources inside Syria and says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
The group has reported hundreds of civilians killed in operations against Daesh in Deir Ezzor and neighboring Raqqa province, where the SDF is fighting with US support to capture the former jihadist bastion Raqqa city.
On Tuesday, the Observatory said a US-led coalition strike in Raqqa killed at least 18 civilians.
The coalition says it takes all measures possible to avoid civilian casualties and that it investigates all credible allegations.
Last month, it acknowledged the deaths of over 700 civilians in its strikes in Syria and Iraq since 2014.
Russia has not acknowledged any civilian deaths in its strikes since its intervened in Syria’s war in 2015 and dismisses the Observatory’s reporting as biased.
The deaths in Deir Ezzor on Wednesday prompted outrage from the opposition Syrian National Coalition which described the incident as a “heinous crime.”
More than 330,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.