Witnesses say some are now being used to terrorise other captives, and are even carrying out killings themselves.
This has not been independently verified but Amnesty International says other girls kidnapped by Boko Haram have been forced to fight.
Boko Haram has killed some 5,500 civilians in Nigeria since 2013.
Two-hundred-and-nineteen schoolgirls from Chibok are still missing, more than a year after they were kidnapped from their school in northern Nigeria.
Three women who claim they were held in the same camps as some of the Chibok girls have told the BBC’s Panorama programme that some of them have 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 to us get ready, that they were going to marry us off.”
She and four others refused.