The UN children's agency and Human Rights Watch appealed Tuesday for the release of more than 100 Kurdish schoolboy and girls kidnapped by jihadists in Syria at the end of May.
The militants from the Islamic State (IS), formerly known as the Islamic State of Iraq and the Levant (ISIL), abducted some 153 schoolchildren on May 29.
The children, who included 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, according to the Syrian Observatory for Human Rights monitoring group.
Another five boys escaped, according to both groups.
UNICEF called for the "immediate and safe return" of those held.
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"These children have nothing to do with the war raging in Syria. They just wanted to finish their exams and go back to their families," said Maria Calivis, regional director for the Middle East and Northern Africa.
"UNICEF expresses its deepest solidarity with the families of the abducted children," Calivis was quoted as saying in a statement.
"Those who commit such crimes must be held accountable."
Human Rights Watch said two of the boys who escaped told the media that ISIL was forcing them to undergo lessons in sharia and jihadist ideology, and that those who misbehaved were beaten.
The group said several of the boys' fathers 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.