It found that at least 3,900 children were killed and 7,300 more maimed due to Boko Haram’s attacks on communities and confrontations with security forces during that period. Suicide attacks became the second leading cause of casualties, which account for over one thousand of child deaths and 2,100 injuries.

Boko Haram, the Islamist group, had waged a campaign of violence and terror across parts of west and central Africa. It has killed more than 20,000 people and displaced 2.3 million others since then.

"With tactics including widespread recruitment and use, abductions, sexual violence, attacks on schools and the increasing use of children in so-called 'suicide' attacks, Boko Haram has inflicted unspeakable horror upon the children of Nigeria's northeast and neighboring countries," Virginia Gamba, the special representative of the secretary-general for children and armed conflict, said in a press release.

The U.N. verified the recruitment and use of 1,650 children by the group, including the use of 90 children for suicide bombings in Nigeria, Cameroon, Chad and Niger. Many of those children were abducted.

In April 2014, Boko Haram militants abducted 276 schoolgirls from the small Nigerian town of Chibok. The majority of those girls are still missing, despite a high-profile international campaign of “#BringBackOurGirls” on social media.

Testimonies from children separated from Boko Haram indicated that other children joined the group due to financial incentives, peer pressure, familial ties and for ideological reasons. Some parents also gave up their children to the group in exchange for security guarantees or economic gain.

Boko Haram fighters then employ the children in "direct hostilities," including for planting improvised explosive devices, to burn schools or houses and in a variety of intelligence-related support roles, the report says.

Schools have been targets of choice for Boko Haram and the U.N. estimates that 1,500 were destroyed since 2014, with at least 1,280 casualties among teachers and students.

The report condemned those violations against children “in the strongest possible terms” and urged all parties to ensure the protection of civilians during armed conflict under international humanitarian, human rights and refugee law.