BEIRUT — At least 16 children are among nearly 30 civilians kidnapped by Islamic State militants in southern Syria a month ago and are being used as a "bargaining chip" in negotiations with the Syrian government and its ally Russia, Human Rights Watch said Saturday.

The rights watchdog said that the children are aged between seven and 15 years old. It called the kidnapping a "war crime," saying the 27 hostages held since July 25 should not be used for bargaining and called for their immediate release. The militants have beheaded a teenage hostage and a woman died in their custody. The reason for her death was not immediately clear. Witnesses told HRW two women managed to escape their abductors.

The government is pressing ahead with an offensive against the group in the southern Sweida province and areas adjacent to Damascus suburbs while locals have formed a negotiating committee to secure the release of their relatives. IS militants have apparently asked for the release of their own members imprisoned by the government. In the video of the teenager's beheading, they called on the government to call off its offensive against the group's remaining holdouts near the capital.

In a multi-pronged attack, the militants raided villages in Sweida, carried out suicide bombings and kidnapped more than 30 people on July 25. The raids set off intense clashes. Witnesses told HRW that the militants had snipers on rooftops and were shooting residents as they fled or tried to drive the wounded to hospitals.

Although largely uprooted from its strongholds in Syria and Iraq, Islamic State militants remain a threat in both countries. The United Nations estimates that between 20,000 and 30,000 fighters remain in the countries, either militarily engaged or hiding among civilians.

The simultaneous attacks in Sweida evoked the dark violent days during the group's heydays in 2014 and 2015, where abductions, beheadings and taking women as sex slaves were commonplace.