At least 27 people are being held by ISIS in southern Syria, Human Rights Watch said Saturday as it deplored the hostage-taking as a "war crime".
The group of mostly women and children were abducted by ISIS during a massive July 25 assault on the Druze community in Sweida, in which the extremists killed more than 250 people.
They are being held by ISIS to use as leverage in negotiations with the Syrian government and its ally Russia, according to HRW.
"Hostage-taking is a war crime," the rights group said.
"Civilian lives should not be used as bargaining chips," said its deputy Middle East director Lama Fakih.
Of more than 30 people taken hostage in the July offensive, at least two have since died.
A 19-year-old male student was beheaded and a video circulated of the killing, which was not released on the militants' usual channels.
Later in August a 65-year-old woman died, with ISIS reporting she had been unwell.
Additionally, two women were able to escape after being abducted from their home, a family member told HRW.
Villagers provided the names of at least 27 people who remain in ISIS captivity, with children as young as seven among them, according to activists in Sweida province.
The province is the heartland of Syria's Druze minority, followers of an offshoot of Shiite Islam who are are despised as heretics by the extremists of ISIS.
ISIS is seeking the release of militants captured by the government in the neighboring province of Daraa, the Britain-based Syrian Observatory for Human Rights said earlier this month.
The extremists currently hold less than three percent of Syria, after losing swathes of territory to government forces backed by Russian firepower.
But ISIS has proven it is still able to launch deadly attacks, with eight pro-government fighters killed and more than 60 wounded in an overnight raid in Sweida province.
The attack brought to 54 the number of pro-government fighters killed in the past month, while 147 extremists have been killed, the Observatory said.
Since the civil war erupted in Syria in 2011, more than 350,000 people have been killed and millions have fled their homes.