Dozens of victims who stayed at camps for the displaced in Borno State's capital Maiduguri told Human Rights Watch they were sexually abused or coerced into sex by camp leaders, vigilante group members, policemen and soldiers. Many of the women were abandoned after becoming pregnant.
"It is bad enough that these women and girls are not getting much-needed support for the horrific trauma they suffered at the hands of Boko Haram," said Mausi Segun, senior Nigeria researcher at HRW.
"It is disgraceful and outrageous that people who should protect these women and girls are attacking and abusing them."
Nigeria's President Muhammadu Buhari is "worried and shocked" by the allegations, his office said in a statement. "Nigerians and the international community can rest assured" that the claims raised in the report "are not being taken lightly," the statement reads.
President Buhari has instructed the Inspector General of Police and other top officials "to immediately commence investigations into the issue," according to the statement.
'I did not tell anyone because I was afraid'
In its report published Monday, HRW documented the accounts of 43 women and girls, who had been displaced from several towns and villages in Borno State.
Eight women were previously abducted by Boko Haram fighters and forced into marriage before escaping to Maiduguri. Four victims told Human Rights Watch they were first drugged before they were raped at the camps. The rights groups said 37 of the victims were coerced into sex through false marriage promises, material and financial help, even food.
A 16-year-old girl from Baga said she was drugged and raped by a vigilante group member, who was in charge of distributing aid at one of the camps.
"I always told him I was too small [young]," she said. "The day he raped me, he offered me a drink in a cup. As soon as I drank it, I slept off."
"I knew something was wrong when I woke up. I was in pain, and blood was coming out of my private part," said the girl. "I did not tell anyone, because I was afraid."
A 30-year-old woman from Walassa fled to a camp with her remaining children, after Boko Haram fighters killed her husband and abducted her young daughters. She said one of the soldiers at the camp approached her for marriage.
"When he was pretending to woo me he used to provide for me, but as soon as I agreed and we began having sex, his gifts began to reduce until he abandoned me," she said. "Now my situation is worse as the pregnancy makes me sick, and I have no one to help me care for my children."
Many victims ashamed
HRW says irregular supplies of food, clothes, medicine and other essentials for the displaced, along with restricted movement are factors contributing to the vulnerability of the victims, who are often widowed women and unaccompanied orphaned girls.
Rape and sexual abuse victims are less likely to seek health care, including psychological counseling, because of the shame they feel, the rights group said. Fewer than a handful of the 43 interviewed women and girls said they had received counseling. Many other victims are suspected to be infected with HIV or other sexually transmitted diseases but are ashamed to seek treatment.
"Failure to respond to these widely reported abuses amounts to severe negligence or worse by Nigerian authorities," said Segun. "Authorities should provide adequate aid in the camps, ensure freedom of movement for all displaced people, safe and confidential health care for survivors, and punish the abusers."
Boko Haram blamed for humanitarian crisis
Years of Boko Haram violence in northern Nigeria have taken a heavy toll on the region and its residents. Efforts by the government and aid groups to alleviate the humanitarian crisis have shown slow progress.
Boko Haram aims to institute Sharia law across the country and is fighting with government forces who are trying to put down its deadly insurgency.
More than 20,000 people have been killed in the violence, thousands abducted and nearly 2.5 million displaced.
In 2014, Boko Haram fighters abducted nearly 300 girls from a school in Chibok, prompting a global campaign for their release. While some of the girls have escaped or have been rescued, many of the remaining girls are believed to be still held by the terror group.