A monitoring group and a leading international rights group said Wednesday that the Kurdish People's Protection Units (YPG) militia and Islamic State of Iraq and al-Sham (ISIS) militants have been using dozens of children in the conflict in Syria. More than 50 child soldiers recruited by ISIS in Syria have been killed since the beginning of this year, the monitoring group, the Syrian Observatory for Human Rights, said.

It said it had documented the deaths of 52 child soldiers, all under the age of 16, who had been part of ISIS's Cubs of the Caliphate program. The program provides intense military and religious training to children in ISIS's areas of control in Syria, the U.K.-based observatory said. As many as 31 were killed in explosions, clashes and airstrikes by the Syrian regime and U.S.-led coalition in July alone. Child soldiers are used to man checkpoints or gather intelligence from areas outside ISIS control, but ISIS has been increasingly using them to execute prisoners or conduct suicide attacks.

So far this year, the extremist group has used 18 children as suicide bombers, most recently in its fight against the YPG in northeastern Syria. "This shows that ISIS is exploiting the suffering of the Syrian people," observatory head Rami Abdel Rahman said. "When a child reaches the point of becoming a suicide bomber, this means that he's been completely brainwashed," Rahman told Agence France Presse (AFP). The observatory said it had received information on dozens more children killed, but that it could not confirm their deaths. Since the beginning of 2015, ISIS has recruited more than 1,100 children.

Also on Wednesday, Human Rights Watch (HRW) criticized the YPG, which is fighting ISIS in northern Syria, of continuing to violate the ban on children in war despite some progress to stop using boys and girls under 18 in combat.

The group signed an agreement last year with the nongovernmental organization Geneva Call, pledging not to send children to war and initially demobilized 149 children in a month starting in June 2014, but both branches of Kurdish forces, the YPG and the Women's Protection Units (YPJ), resumed using boys and girls under 18 as fighters, New York-based HRW said in a report.

"The YPG promised to stop sending children to war, and it should carry out its promise," said Fred Abrahams, HRW special adviser. "Of course the Kurdish forces are fighting groups like ISIS that flout the laws of war, but that's no excuse to tolerate abuses by its own forces." HRW has compiled a list of 59 children, 10 of them under 15, who were allegedly recruited by or volunteered for YPG or YPJ forces, it said. Some of the children were recruited without the consent of their parents.

International humanitarian law prohibits armed groups from enlisting children younger than 15 years of age or using them to actively participate in hostilities. HRW said an additional concern was the Kurdish group's creation of a "non-combatant category" for children aged 16 and 17, urging it to stop recruiting children even if they are not serving a military function.