The children’s abuse amounted to war crimes, HRW said, quoting testimony from interviews with four boys among the group.

The children described being forced to pray five times a day and undergoing intense religious instruction, as well as being forced to watch videos of ISIL in combat and beheading captives, the New York-based rights group said.

"Those who didn’t conform to the program were beaten. They beat us with a green hose or a thick cable with wire running through it. They also beat the soles of our feet," the report quoted one boy as saying. "They sometimes found excuses to beat us for no reason ... They made us learn verses of the Quran and beat those who didn’t manage to learn them."

The boys said the only reason given for being released was that their religious education had been completed. The last children to be released are seeking shelter in Turkey, the HRW said.

Captives from families with members fighting with the Kurdish militia, called YPG — which has been defending Kobane — were singled out for abuse, the children said.

Their captors, who came from Syria, Jordan, Libya, Tunisia and Saudi Arabia, "told them to give them the addresses of their families, cousins, uncles, saying, 'When we go to Kobane, we will get them and cut them up.' They saw the YPG as infidels,” one 15-year-old boy told HRW.

Other Kurdish children and adults are still in captivity, the rights group said. ISIL is also thought to hold several Western hostages and foreign journalists.

The United States and its allies have pounded ISIL targets in Iraq and Syria since August, stepping up the bombardment after ISIL fighters moved in on Kobane in October.

Al Jazeera and wire services