Beirut: IS 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.
The 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 head Rami Abdulrahamn.
Deir Al Zor is the main town in a province of the same name. The province links IS's de facto capital in the Syrian city of Raqqa with territory controlled by the militant group in neighbouring 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 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".
A source close to the Syrian government side said on Saturday that some of those killed had been beheaded.
Reuters was unable to independently verify the reports.
IS 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 Al Zor in 2014.
The group, in control of most of Deir Al Zor province, has laid siege since March on remaining government-held areas in the city of Deir Al Zor.
Residents are facing severe food shortages and sharply deteriorating conditions. Of those under siege in the city, 70 per cent are women and children, and many have been displaced from their homes elsewhere and are living in temporary shelters.
Meanwhile, the death toll from Saturday's air strikes on the Syrian city of Raqqa, de facto capital of the IS group, has risen to 40 civilians, including eight children, the Syrian Observatory for Human Rights said on Sunday.
The group said it remained unclear whether the warplanes which carried out the strikes were Russian or from the US-led coalition.
The Observatory, which on Saturday had put the death toll from the strikes at 16 people, said they had hit shops and small restaurants in Raqqa.