A Syrian rights monitor has reported that the Islamic State (IS) militant group has recruited some 400 children to its ranks in Syria since the beginning of the year.

The Britain-based Syrian Observatory for Human Rights (SOHR) said Tuesday that the Sunni Muslim group has focused on recruiting children around mosques, schools, and public areas where executions by stoning, beheading, crucifixion, and other brutal acts of violence are carried out on non-conforming individuals.

"They use children because it is easy to brainwash them. They can build these children into what they want, they stop them from going to school and send them to IS schools instead," Rami Abdulrahman, the head of SOHR, told Reuters.

IS also allegedly bribes parents with money to relinquish their sons, who are then sent to military camps where they are taught aspects of Sharia — an Islamic religious law — as practiced and interpreted by the group. The children are also given weapons and combat training, SOHR said.

IS has involved children in its violent campaign to establish a caliphate straddling across Syria and Iraq since it first launched its offensive in the summer of 2014. The group has sought to indoctrinate the children and normalize extreme violence and murder to them at an early age through specialized schools and training camps where they are allegedly taught to kill.

The group has previously released propaganda video depicting the so-called "cubs of the caliphate" training in these types of camps, as well as executing prisoners the group has taken hostage.

In February, terror monitors SITE Intelligence Group reported that IS was distributing flyers advertising the opening of English language schools for the children of foreign fighters in the group's de-facto Syrian capital of Raqqa.

The flyers emerged the same week the militant group released a propaganda video showing young boys dressed in fatigues and black headbands performing synchronized drills at a Raqqa camp. Some of the boys appearing in the video are as young as 4 or 5 years old.

The United Nations' children's fund told VICE News today that while it could not independently verify the numbers of children IS recruited between January and March 23, the organization is "aware of the phenomenon" and is monitoring the situation.

"It's important to remember the recruitment and use of children in combat has been happening by all parties to the conflict," Juliette Touma, Syria crisis spokeswoman for UNICEF's Middle East office said. "Children have not only been used to fight, but also to man checkpoints, and in support jobs like cooking or cleaning, or treating injured fighters. These are just a variety of ways that children have been exploited in the conflict."