At least 75 people were killed by a car bomb as they attempted to flee the city of Deir-ez-Zor, a Britain-based monitor said on Saturday.
The Syrian Observatory for Human Rights said an Islamic State (ISIS) car bomb killed dozens of people displaced by fighting as they gathered on the east side of the Euphrates River.
"The Islamic State group targeted displaced people with a car bomb on the eastern bank of the Euphrates River, killing dozens and wounding dozens of others," Observatory head Rami Abdel Rahman said.
Abdel Rahman said the victims had fled battlegrounds in the province where the United States-backed Syrian Democratic Forces alliance is also on the offensive against the hard-line group.
The SDF is attacking ISIS positions on the eastern bank of the Euphrates which cuts across the province, while Russian-backed Syrian forces are battling the jihadists on the western bank of the river.
The parallel offensives have sent thousands of civilians fleeing for their lives, some straight into the desert.
Opposition activists and local council members said on Friday that the SDF had prevented hundreds of people from fleeing across the Euphrates to areas under its control.
Sonia Khush, Syria director for Save the Children, said an estimated 350,000 people have fled the recent fighting in Deir-ez-Zor province, half of them children.
"The situation in the city, and surrounding countryside, has been especially bleak with civilians trapped between the fighting and all too often caught in the crossfire," she said.
Despite being driven out from large parts of Deir-ez-Zor, ISIS still controls 37 percent of the province with its fighters deployed mostly on the eastern side.
Deir-ez-Zor, which lies on the west bank of the Euphrates River, is the largest and most important city in eastern Syria, and is the center of the country’s oil production industry.
This article was translated and edited by The Syrian Observer. Responsibility for the information and views set out in this article lies entirely with the author.