Islamic State has released 270 out of 400 civilians kidnapped by the group earlier last week near the eastern Syrian city of Deir al-Zour, a monitoring group said Wednesday.
The Syrian Observatory for Human Rights said most of the released were women, children under the age of 14 and men over 55.
The Britain-based group added that the terrorist militia had kept male prisoners whose ages vary between 15 and 55 for more questioning.
On Sunday, Islamic State fighters abducted at least 400 people from the regime-held village of al-Baghaliyeh following a major assault on the area.
According to the Observatory, 50 new people were rounded up by the group in the past 24 hours in raids carried out on houses in the same village.
"We fear for the safety of the remaining group of prisoners; and we still do not know for sure if there are still women in captivity," the head of the Observatory Rami Abdel-Rahman told dpa.
Islamic State controls most of the Deir al-Zour province and its capital city of the same name.
The al-Qaeda breakaway group has been besieging regime-held areas in Deir al-Zour for more than a year.
In recent weeks, Islamic State strongholds in Syria have been the target of intensified airstrikes separately mounted by a US-led alliance and Russia.
More than 250,000 people are estimated to have been killed in Syria's conflict since it started in 2011, according to the UN.