Boko Haram militants have abducted upwards of 2,000 girls in the past two years, some of whom have been forced to become child soldiers alongside abducted boys, reports the human rights group, Amnesty International.
In a new, 90-page study of atrocities committed by Boko Haram fighters, Amnesty estimates that they have killed at least 8,300 people, mostly civilians, since 2013. In addition they have forced more than 1.2 million people from their homes and made hundreds of thousands destitute.
"The situation seems to have spiralled out of the government's control as the human rights violations and abuses committed by parties to the conflict have become more sophisticated and deadly," Amnesty says.
The Amnesty report is being released on the first anniversary of the abduction of 276 girls from the Government Girls' Secondary School in Chibok, Borno state.
It quotes a senior Nigerian military source as saying that the girls have been split into three or four groups and are being held in Boko Haram camps - some in the Sambisa forest, others around Lake Chad and others near the Gorsi mountains in Cameroon.
The study was unable to verify reports that some of the Chibok girls have been forced into marriage, but Amnesty says it has received other reports of women being forced to marry Boko Haram fighters.
The report quotes girls previously abducted as saying they had been held with other girls, including some from Chibok, in houses in Gwoza, in Borno near the Cameroonian border. It reports one girl abducted in October 2014, identified only as "Mary", as testifying: "I saw several of the abducted girls going to battlefield."
A witness named by Amnesty as "Aisha Yusuf" (not her real name) is quoted as saying that Boko Haram "used to train girls how to shoot guns. I was among the girls trained to shoot. I was also trained how to use bombs and how to attack a village. They'll dress us and demonstrate to us how to explode a bomb. This training went on for three weeks after we arrived. Then they started sending some of us to operations. I went on one operation to my own village."
Amnesty also reports receiving information that both boys and girls under 15 years old have been forced to take an active part in battle and in executions.
The rights group says this amounted to the war crime of "conscription or recruitment of child soldiers", and Boko Haram ought to be investigated for the offence. Arising from Boko Haram's treatment of women, Amnesty also calls for its members to be investigated for the war crimes of rape, sexual slavery and other forms of sexual violence.
"Amnesty International's research has led to the conclusion that Boko Haram has committed both war crimes and crimes against humanity in north-east Nigeria," the report says.
The report is based on four research missions in 2014 and 2015 and on telephone interviews with eyewitnesses and victims carried out after Boko Haram attacks or after witnesses escaped. It focuses on Boko Haram's atrocities; Amnesty says it will issue a report later on human rights violations by Nigerian government forces fighting Boko Haram.