(New York) – The Islamic State of Iraq and Sham (ISIS) should immediately release the estimated 133 Kurdish boys it has held hostage in northern Syria for a month.

ISIS abducted 153 children, ages 13 and 14, from the mostly Kurdish town of Ain al-`Arab (Kobani in Kurdish) on May 29, 2014, as they returned from taking year-end exams in the city of Aleppo. Five boys escaped and ISIS released 15 others on June 28, apparently in return for the release of three ISIS members held by Kurdish forces.

Click to expand Image Person waives an Islamic State of Iraq and Sham(ISIS) flag in Raqqa, Syria, on June 29, 2014. © 2014 Reuters

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.

“ISIS abducted these children as they returned from taking an exam,” said Fred Abrahams, special advisor at Human Rights Watch. “To hold them hostage is both cruel and a flagrant violation of the laws of war.”

Taking of hostages and using, conscripting and enlisting children in an armed conflict are war crimes.

Human Rights Watch spoke with the fathers of three of the abducted boys, as well as three local officials from the ruling political party in Ain al-`Arab, the Kurdish Democratic Union Party (PYD).

All three fathers said that a large group of male and female students had travelled on May 29 to Aleppo, 110 kilometers from Ain al-`Arab, to take their official school exams because the Syrian government was not offering the exam in their town. The journey required the children to pass through territory controlled by ISIS.

The top education official in Ain al-`Arab, Hussein Mohammad Ali, told Human Rights Watch that at least 1,000 students, ages 13 to 18, travelled to Aleppo in buses and mini-buses, along with some teachers. ISIS allowed the convoy to proceed to Aleppo, but stopped the first group that returned – 13 and 14 year-olds from the ninth grade – in the ISIS-controlled town of Manbij, Ali and the two other local officials said. There ISIS fighters separated the boys from the girls and sent the girls home with the drivers.

About 100 girls from the class returned to Ain al-`Arab, Ali said. The rest of the children stayed in Aleppo and eventually returned safely to Ain al-`Arab via other routes, but 153 of the ninth-grade boys were forced to remain in Manbij.

“We found out they were kidnapped the next day [May 30] from the families of the girls who were let go,” the father of an abducted 13-year-old boy said. “The girls were let go with one of the bus drivers who drove them to a civilian house where they rested and called their families.”

A few days later, two of the abducted boys managed to escape, the fathers and local officials said. The boys told local officials that ISIS was holding the other children in Manbij and giving them adequate water and food.

One of the boys who escaped told CNN that ISIS made the group wake early every morning for prayer and then lessons in Sharia. At night ISIS fighters preached to them about radical jihadist ideology and showed the children violent videos of suicide bomb attacks and executions.

Human Rights Watch recently documented that ISIS has recruited children to fight along with adults, and asked children to participate in suicide attacks, though it is not clear if that is happening in this case.

“We were all so scared,” said one of the two boys who escaped, identified as Mohammad. “On the way back, we were celebrating that we had finished our tests. We were excited to go home and see our families. We didn't know why they took us.”

The other boy who escaped told the Guardian that ISIS beat the boys who misbehaved. “Ten boys were beaten every day,” he said. “But most of us were well-behaved, to not get beaten.” The boy also said that ISIS showed the children violent videos from Iraq of “people being slaughtered.”

The fathers of two of the boys still held in Manbij told Human Rights Watch that they had received phone calls from their sons a few days after the abduction, and once every week since then. “He told me that he is okay and they are letting them play in the playground,” the father of a 14-year-old said. “They are giving them food and a place to sleep.”

The third father said he had not spoken with his son. “The rest of the parents are calling their children but my son did not call me and I tried to call but nobody answered,” he said. “I am worried about my son.”

Local officials in Ain al-`Arab said that negotiations with ISIS are ongoing to secure the boys’ release. CNN reported that ISIS wants the PYD’s armed wing, the People’s Protection Units (YPG), to release captured ISIS members in exchange for the children. “We are waiting for their demands to determine if there should be a prisoner swap,” the YPG spokesman, Redur Xelil, told McClatchy.

According to media reports and the Syrian Observatory for Human Rights, on June 28 ISIS released 15 of the children. A Kurdish news agency published the names of the fifteen freed boys and reported that the YPG had released three ISIS fighters in return.

The town of Ain al-`Arab and surrounding area is one of three regions in northern Syria the PYD controls. The other two regions are `Afrin (Êfrîn) and Jazira (Cezire). In January 2014, the PYD and allied parties established a transitional administration in the three regions.

ISIS is fighting the YPG around Ain al-`Arab and has at times cut electricity and water to the area.

On May 29, ISIS forces entered the village of al-Taliliya in the Jazira region and executed at least 15 civilians, including six children. The victims were Syrian Arabs who had been displaced from the countryside around Aleppo.

“The kidnapping of Kurdish students is a reminder of the heavy toll paid by Syria’s children and that ISIS abuses are not limited to Iraq,” Abrahams said.