Islamic State militants kidnapped at least 400 civilians when they attacked government-held areas in the eastern Syrian city of Deir al-Zor on Saturday, a monitoring group said, Reuters reports.
The Syrian Observatory for Human Rights said on Sunday families of pro-government fighters were among those abducted.
Syria's state news agency SANA said earlier that at least 300 people, including women and children, had been killed during the attacks in Deir al-Zor, but it made no mention of people getting kidnapped.
Syria's government condemned the killings which it described as a "horrific massacre against the residents of Begayliya in Deir al-Zor".