Beirut - Jihadists from the Islamic State must free some 133 Kurdish schoolboys kidnapped in Syria a month ago, Human Rights Watch said on Tuesday.
The group, formerly known as the Islamic State of Iraq and the Levant, kidnapped some 153 schoolchildren on 29 May.
The children, mostly boys but including around 10 girls, were on their way back from taking year-end school exams in the northern city of Aleppo.
They were heading to the town of Ain al-Arab on buses when they were stopped in the IS-controlled town of Manbij.
Since their capture, IS has released around 15 children, including all the girls that had been held, according to the Syrian Observatory for Human Rights monitoring group.
Another five boys escaped, according to both groups.
"Two of the boys who escaped told the media that ISIS was forcing the children to undergo lessons in Sharia and jihadist ideology, and one of these boys said that Isis beat the children who misbehaved," HRW said.
The group said several fathers of the kidnapped boys had received regular phone calls from their sons, though others said they had not had any contact.
There have been fears that IS could train the boys to fight, though there was no evidence of that yet.
"Taking of hostages and using, conscripting and enlisting children in an armed conflict are war crimes," HRW said.
IS is fighting Kurdish factions in parts of Syria, and has regularly taken Kurdish civilians hostage, occasionally freeing them in exchange for the release of their fighters held by Kurdish groups.