Activists from the Syrian Observatory for Human Rights stated that ISIS fighters allegedly abducted around 400 civilians, including women and children, as well as family members of pro-regime fighters, during the extremist group's siege on Syria's eastern city of Deir Ezzor on Saturday, according to The Daily Mail.

The human rights group further believes that the kidnappings were executed after the extremist group attacked a number of government-held locations north of the city. 

Rami Abdulrahamn, head of the Observatory, states that with ISIS' reputation of brutality and barbarity, the lives of the abducted are in grave danger, reports Yahoo! News.

"There is genuine fear for their lives, there is a fear that the group might execute them as it has done before in other areas," he said.

After the abduction, the civilians were allegedly taken to areas that are under the radical group's control. During the siege in the city, about 85 civilians and 50 regime forces were also killed, prompting Syria's state news agency, SANA, to refer to the events as a "massacre," according to The Saudi Gazette.

If the reports are indeed true, the fatalities and the abductions would equal one of the highest tolls for a single day in the country's nearly five-year war.

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