Human Rights Watch based its conclusions on interviews with several children who were among more than 150 Kurdish boys from Kobani abducted in late May as they were returning home after taking school exams in the city of Aleppo. It said around 50 of the Kurds escaped early in their captivity, while the rest were released in batches.
Four of the children who were released told the rights group that they were held by the terrorists in the northern Syrian town of Manbij.
They described frequent abuse at the hands of the militants, who used a hose and electric cable to administer beatings.