Women who claim they lived in the same camps as some of the 219 Nigerian girls who were taken from their school in the town of Chibok last April told the BBC that many are now administering punishments on behalf of Boko Haram.
Those punishments include flogging young girls who were unable to recite from the Quran, and slitting the throats of captured men.
"The abduction and brutalisation of young women and girls seems to be part of the modus operandi of Boko Haram," The Guardian quoted Africa director at Amnesty International Netsanet Belay as saying.
Boko Haram, which means 'Western education is forbidden', launched a rebellion in 2009 and seeks to impose a caliphate on Nigeria - Africa's largest nation.
Militants abducted 276 girls from their secondary school in north-east Nigeria in April 2014, with 57 later escaping. The remaining 219 have not been seen since a video was released the following month apparently showing about 130 of them reciting from the Quran.
'Miriam', 17, told the BBC that she met some of the Chibok girls while she was held captive for six months in a Boko Haram camp, although they were kept in a separate house to the others. She said they were "brainwashed" by the Islamist militants.
Miriam claimed that some of the Chibok girls had killed several Christian men in her village.
Anna, 60, said she fled a Boko Haram camp in the Boko Haram stronghold of the Sambisa forest, where she too had seen the Chibok girls commit murder.
"People were tied and laid down and the girls took it from there. The Chibok girls slit their throats," she said.
"They shared the girls out as teachers to teach different groups of women and girls to recite the Quran," she said.
"Young girls who couldn't recite were being flogged by the Chibok girls."
Amnesty says that at least 2,000 women and girls have been abducted by Boko Haram since the beginning of 2014, with many targeted because they are Christians or attending school.
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