Islamic State militants kidnapped at least 400 civilians when they attacked government-held areas in the eastern Syrian city of Deir-ez-Zor on Saturday, a monitoring group said.
The Britain-based Syrian Observatory for Human Rights said on Sunday families of pro-government fighters were among those abducted.
"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," said the Observatory's director Rami Abdul Rahamn.
Deir-ez-Zor links the Islamic State's de facto capital in the Syrian city of Raqqa with territory controlled by the militant group in neighboring Iraq.
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-ez-Zor, but it made no mention of any abductions.
Syria's government condemned the killings which it described as a "horrific massacre against the residents of Begayliya in Deir-ez-Zor".
A source close to the Syrian government said on Saturday that some of those killed had been beheaded. Reuters was unable to independently verify the reports.
ISIS has previously carried out mass killings following military assaults in Iraq and Syria, including the slaughter of 200 soldiers captured from the Tabqa airbase in Raqqa province, and hundreds of members of the al-Sheitat tribe in Deir-ez-Zor in 2014.
The group, which controls most of Deir-ez-Zor province, has laid siege on remaining government-held areas in the city of Deir-ez-Zor since March 2014.
Residents are facing severe food shortages and sharply deteriorating living conditions. Of those under siege in the city, 70 percent are women and children, and many have been displaced from their homes elsewhere and are living in temporary shelters.
This article was edited by The Syrian Observer. Responsibility for the information and views set out in this article lies entirely with the author.