Girls and women abducted by the Nigerian Islamist group Boko Haram have described life in captivity which includes forced marriage and labour, rape, torture, psychological abuse and coerced religious conversion.

More than 500 women and girls have been seized and held in militant camps since 2009, including 60 reportedly kidnapped from two towns in north-eastern Nigeria last week. Many have been targeted because they are Christians or attending school.

The single biggest abduction was of 276 girls from a school in Chibok in April. The kidnappings triggered worldwide protests and military assistance from western governments, but 219 girls are still missing.

Testimonies from girls who escaped their captors – along with others abducted in separate incidents who fled or were released –and gathered by Human Rights Watch have built a picture of violence and terror inside Boko Haram’s camps.

Its report, Those Terrible Weeks in Their Camp: Boko Haram Violence Against Women and Girls in North-east Nigeria, is based on interviews with 30 women and girls abducted between April 2013 and April 2014, plus 16 people who witnessed kidnappings. It calls on the Nigerian government to adopt stronger measures to protect women and girls, provide help for victims and prosecute those responsible.

The women and girls were held in eight different camps for periods ranging from two days to three months, after being taken from their homes, while working on farms, fetching water or at school. The report said that abducted women and girls held by Boko Haram ranged in age from infancy to 65.

Facebook Twitter Pinterest The Chibok school from where more than 200 schoolgirls were abducted by Boko Haram Islamists. Photograph: STR/AFP/Getty Images

“They and many others they saw in the camps were subjected to physical and psychological abuse; forced labour; forced participation in military operations, including carrying ammunition or luring men into ambush; forced marriage to the captors; and sexual abuse, including rape,” the report says. “In addition, they were made to cook, clean and perform other household chores. Others served as porters, carrying the loot stolen by the insurgents from villages and towns they had attacked.”

A 15-year-old girl who was held in a Boko Haram camp for four weeks in 2013 described being forced to marry a militant more than twice her age. “After we were declared married I was ordered to live in his cave but I always managed to avoid him. He soon began to threaten me with a knife to have sex with him, and when I still refused he brought out his gun, warning that he would kill me if I shouted. Then he began to rape me every night. He was a huge man in his mid-30s and I had never had sex before. It was very painful and I cried bitterly because I was bleeding afterwards.”

A social worker who has dealt with victims of Boko Haram violence told HRW that the rape of abducted women had been underreported because of the stigma and shame attached to sexual abuse in northern Nigeria.

A 19-year-old who was raped said: “I could not tell anyone what happened, not even my husband. I still feel so ashamed and cheated.” Another woman with her, who was also raped, “vowed never to speak of it again as she was single and believes that news of her rape would foreclose her chances of marriage”.

The majority of abductions by Boko Haram were of Christian women and girls, and many of HRW’s interviewees described being threatened with death or violence if they refused to convert to Islam. One woman said: “I was dragged to the camp leader who told me the reason I was brought to the camp was because we Christians worship three gods. When I objected to his claim, he tied a rope around my neck and beat me with a plastic cable until I almost passed out. An insurgent who I recognised from my village convinced me to accept Islam lest I should be killed. So I agreed.”

Facebook Twitter Pinterest Newspapers with various front page headlines on the Chibok girls and their possible release after news of a ceasefire with Boko Haram. Photograph: STRINGER/REUTERS

A month after the Chibok kidnap, a video was released by Boko Haram, showing about 130 girls wearing Islamic dress. Two were singled out to say they had converted to Islam.

Some abducted women and girls described forced labour and participation in military operations. A 19-year-old said: “I was told to approach a group of five men we saw in a nearby village and lure them to where the insurgents were hiding.” She told the young men that she needed help. “When they followed me for a short distance, the insurgents swooped on them. Once we got back to the camp, they tied the legs and the hands of the captives and slit the throats of four of them as they shouted ‘Allahu Akbar’. Then I was handed a knife to kill the last man. I was shaking with horror and couldn’t do it. The camp leader’s wife took the knife and killed him.”

Many of those interviewed by HRW showed signs of stress and anguish, according to the report, although only the Chibok girls had been offered limited counselling. One 15-year-old girl said: “I could not stop crying even when the insurgents threatened to kill me if I did not keep quiet. I kept on thinking, is it not better to die now than to face whatever terrible things they could do to me when we get to their camp? Even after I escaped from them and live far away from my village, I am still afraid. I think of death many times. My father tries. He encourages me to forget everything, but it is not easy for me. I have terrible dreams at night.”

All of those interviewed by HRW said more could have been done by government security forces to prevent abductions and respond more quickly when they happened. The organisation calls on the Nigerian authorities to investigate and prosecute those who commit serious crimes, to protect schools and the right to education, and ensure access to medical and mental health services for victims of abductions.

“The Chibok tragedy and #BringBackOurGirls campaign focused much-needed global attention to the horrific vulnerability of girls in northeastern Nigeria,” said Daniel Bekele of HRW. “Now the Nigerian government and its allies need to step up their efforts to put an end to these brutal abductions and provide for the medical, psychological and social needs of the women and girls who have managed to escape.”

Inadequate response by the Nigerian government had left many in fear, he added. “The government and its allies need to step up their protection, support services, and prosecutions of abuses to stop this cycle of terror.”

Ten days ago, Nigeria said that Boko Haram had agreed to a ceasefire, which would lead to talks on releasing some or all of the Chibok girls. But the militant organisation did not confirm any deal, and the statement was met with some scepticism by analysts.