Islamic State (Isis) militants kidnapped at least 400 civilians in a raid on government-held areas of the city of Deir al-Zour, monitoring group the Syrian Observatory for Human Rights said. The group said that during the attack on Saturday, 16 January, the families of pro-government fighters had been seized.
"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," the Observatory's director Rami Abdulrahamn told Reuters.
The city is the capital of a province bearing the same name, which Isis (Daesh) largely controls, and links the group's de facto capital in Raqqa with territory it controls in neighbouring Iraq.
Syrian state news agency SANA earlier reported that up to 300 civilians, including women and children, had been killed in the raid, but did not report the kidnappings. A source close to the Syrian government told Reuters that some of those killed in the massacre had been beheaded and crucified.
The Observatory put the figure killed lower, and said that 135 people were killed in the attack, consisting of 85 civilians and 50 pro-government fighters. It said that those abducted were seized from the suburb of Baligia and other areas in the north-west of the city and taken to Raqqa.
IS launched its attack using suicide bombers followed by a ground assault, with the Syrian army responding with artillery fire.
There is concern over the escalating humanitarian crisis in government-held areas of the besieged city, which IS controls more than 60% of, with the UN warning there is a dire shortage of food and widespread malnutrition.
Russia has conducted airstrikes on Isis positions in the city and airdropped limited amounts of food to the estimated 200,000 civilians trapped.