The U.N. on Friday said Boko Haram had conscripted 140 children as fighters and suicide bombers over three years in its violent activities in Nigeria's northeast.

UNICEF Country Representative Mohammed Fall said in Maiduguri, Boko Haram's birthplace, that the violent group is committing crimes against humanity and war crimes.

"From our records, 140 children -- mostly girls -- have been used by Boko Haram in the northeast in three years," Fall said. He said 80 children had been used by the group this year.

"This is worrisome and Boko Haram should stop recruiting and using children to carry out attacks," Fall said while signing an action plan between the UN and local security volunteer group, the Civilian JTF, on the use of children in armed conflict.

Boko Haram's insurgency has left at least 20,000 people dead and displaced more than 2.6 million others since it began in 2009.

Children have been particularly affected, with abductions and forced recruitment widespread, as well as attacks on schools teaching the so-called "Western education" the group despises. The U.N. report said at least 3,900 children were killed and 7,300 others injured as the humanitarian situation worsened in northeast Nigeria between January 2013 and December 2016.