The Chibok schoolgirls taken by Boko Haram last year have been "brainwashed" and are carrying out orders to kill on behalf of their captors, witnesses have claimed.

Speaking to the BBC, two women who have fled captivity by the Islamic militant group in Nigeria said they witnessed a number of the former schoolgirls committing murder.

One young woman was held by Boko Haram for six months. She managed to escape, but at just 17 years old she was forced to marry a militant and is now pregnant. She told the BBC that she met some of the girls taken from Chibok, and that they lived separately to the other captives. She described them as being "brainwashed".

"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. She also recalled a time that the girls killed several men in her village.

"They were Christian men. They [Boko Haram militants] forced the Christians to lie down. Then the girls cut their throats."

Another 16-year-old escapee said she saw a Chibok schoolgirl while she was held captive. "She was just like any of the Boko Haram wives," she said. "We are more scared of the wives than the husbands."

A 60-year-old woman, Anna, who was held hostage by Boko Haram for five months told the BBC that the Chibok schoolgirls she met carried guns, and were described as "teachers" by militants.

"They shared the girls out as teachers to teach different groups of women and girls to recite the Koran. Young girls who couldn't recite were being flogged by the Chibok girls," she explained. Anna also said she had seen the schoolgirls commit murder.

Netsanet Belay, Africa director at Amnesty International, said that the abduction and brutalisation of young women and girls "seems to be part of the modus operandi of Boko Haram".

Dr Fatima Akilu, of Nigeria's counter-violence and extremism programme, said she hadn't seen signs of radicalisation among the 300 rescued women and children she is currently looking after. "But if it did occur we would not be surprised," she told the BBC.

"In situations where people have been held, there have been lots of stories where they have identified with their captors."

Young girls have been used as suicide bombers by Boko Haram, and there are now increasing fears that some of the abducted Chibok schoolgirls might be among them. Yusuf Mohammed, who works with young people affected by trauma in Maiduguri, told The Daily Beast: "Militants feel it is easier to intimidate and brainwash young girls than adult women. Besides, these girls come cheap, and most of them are extremely loyal."

Despite an international campaign and promises that the girls would be released, 219 of those taken form their school in Chibok remain in captivity. They were abducted on April 14 last year.