At least 27 people are being held by the Islamic State group 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 IS during a massive July 25 assault on the Druze community in Sweida, in which the jihadists killed more than 250 people.
They are being held by IS 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 jihadists' usual channels.
Later in August a 65-year-old woman died, with IS reporting she had been unwell.