Some of the 219 Chibok schoolgirls kidnapped on April 14, 2014 have been forced to join the fighters of the Boko Haram sect, the BBC has been told.
Witnesses told BBC Panorama Programme that some of them are now being used to terrorise other captives, and are even carrying out killings themselves.
The BBC cannot verify the testimony, but Amnesty International says other girls kidnapped by Boko Haram have been forced to fight.
The Chibok schoolgirls are still missing, more than a year after they were kidnapped from Government Secondary School, Chibok, Borno State.
Three women who claimed they were held in the same camps as some of the Chibok girls told the BBC?s Panorama Programme on Monday that some of them had been brainwashed and are now carrying out punishments on behalf of the militants.
Seventeen-year-old Miriam (not her real name) fled Boko Haram after being held for six months. She was forced to marry a militant, and is now pregnant with his child.
Recounting her first days in the camp she said, ?They told us to get ready, that they were going to marry us off.?
She and four others refused.
?They came back with four men, they slit their throats in front of us. They then said that this will happen to any girl that refuses to get married,? she stated.
Faced with that choice, she agreed to marry, and was then repeatedly raped. ?There was so much pain,? she said. ?I was only there in body, I couldn?t do anything about it.?
While in captivity, Miriam described meeting some of the Chibok schoolgirls. She said they were kept in a separate house to the other captives.
?They told us: ?You women should learn from your husbands because they are giving their blood for the cause. We must also go to war for Allah.??
She said the girls had been ?brainwashed? and that she had witnessed some of them kill several men in her village.
?They were Christian men. They [the Boko Haram fighters] forced the Christians to lie down. Then the girls cut their throats.?
It is not possible to independently verify Miriam?s claims, but human rights group, Amnesty International said their research also showed that some girls abducted by Boko Haram had been trained to fight.
?The abduction and brutalisation of young women and girls seems to be part of the modus operandi of Boko Haram,? said Netsanet Belay, Africa director, research and advocacy at Amnesty International.
Anna, aged 60, is one of them. She fled a camp in the Sambisa Forest in December where she was held for five months. She now sits beneath a tree close to the cathedral in the Adamawa State capital of Yola. Her only possessions are the clothes she ran away in.
She said she saw some of the Chibok schoolgirls just before she fled the forest.
?They had guns,? she said.
When pressed on how she could be sure that it was the Chibok schoolgirls that she?d seen, Anna said, ?They (Boko Haram) didn?t hide them. They told us, ?These are your teachers from Chibok.?
?They shared the girls out as teachers to teach different groups of women and girls to recite the Koran,? Anna recalled.
She added, ?Young girls who couldn?t recite were being flogged by the Chibok girls.?
Like Miriam, Anna also said she had seen some of the Chibok schoolgirls commit murder. ?People were tied and laid down and the girls took it from there? The Chibok girls slit their throats,? said Anna.
Anna said she felt no malice towards the girls she had seen taking part in the violence, only pity. ?It?s not their fault they were forced to do it,? she added.
?Anyone who sees the Chibok girls has to feel sorry for them,? he said.