More than 50 child soldiers recruited by the Islamic State group in Syria have died this year, the Syrian Observatory for Human Rights said on Wednesday. The children, recruited under the militant group’s “cubs of the caliphate” program, were all below the age of 16 years, according to media reports.

“This shows that Daesh [ISIS] is exploiting the suffering of the Syrian people,” Rami Abdel Rahman, director of the U.K.-based agency, told Agence France-Presse (AFP).

ISIS, which has expanded its presence across northern Syria and Iraq, has recruited children as young as 10 years -- many of whom have been filmed executing prisoners. In a video released earlier this year, dozens of pre-teen boys were shown being indoctrinated in ISIS-run “schools” and trained to fight. In an earlier video, released in November, several Kazakh children wearing military fatigues were also shown being trained in ISIS camps.

Although the exact number of child soldiers currently fighting for the militant group is hard to estimate, since the beginning of this year, ISIS is believed to have recruited more than 1,100 children.

“When a child reaches the point of becoming a suicide bomber, this means that he's been completely brainwashed,” Abdel Rahman told AFP.

In addition to ISIS, militants of the Jabhat al-Nusra -- al Qaeda’s Syrian affiliate -- have also been accused of recruiting child soldiers in the past.

“In 77 per cent of the cases, children were armed or used in combat and almost one fifth were under 15 years of age. Boys associated with armed groups were commonly between 14 and 17 years of age, with 17 verified cases under the age of 15,” the United Nations said, in a report released last month. “In many cases, children were paid to fight for salaries of up to $400 per month. … The payment of relatively large salaries created an incentive for children and their parents under difficult economic circumstance.”

Also on Wednesday, the Human Rights Watch accused Kurdish fighters in northern Syria of using child soldiers in its fight against ISIS.

“The YPG [Kurdish People's Protection Units] promised to stop sending children to war and it should carry out its promise,” Fred Abrahams, special adviser at Human Rights Watch, said, in a statement released Wednesday. “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.”

Kurdish fighters and their allies -- aided by U.S.-led airstrikes -- have been battling ISIS militants across cities in northern Syria, including near Raqqa -- the capital of the militant group’s so-called caliphate.