Human Rights Watch made the claim as it outlined testimony from dozens of former hostages who documented physical and psychological abuse at the hands of the militants.
Suspected Islamist fighters kidnapped about 30 children, including girls as young as 11, in Borno state at the weekend. A week earlier, at least 40 women and girls were seized in neighbouring Adamawa.
Both kidnappings - and continued violence in northeast Nigeria and northern Cameroon - have cast doubt on government claims of a ceasefire deal and agreement for the release of 219 schoolgirls held since April.
"I was told to hold the bullets and lie in the grass while they fought. They came to me for extra bullets as the fight continued during the day," she said.
"When security forces arrived at the scene and began to shoot at us, I fell down in fright. The insurgents dragged me along on the ground as they fled back to camp."
In another operation, she said she was handed a knife to kill one of five captured civilian vigilantes brought to one of the camps and summarily executed.
A wave of attacks by female suicide bombers earlier this year prompted speculation that Boko Haram may have been using abducted women and young girls to carry out attacks.
But there has been no concrete evidence to prove whether the attackers were kidnap victims who were coerced or volunteers.
In July, a 10-year-old was detained in Katsina state, northwest Nigeria, and found to be strapped with explosives.
In all, 30 women and girls between April 2013 and April this year were interviewed, including 12 of the 57 who fled when the militants raided a school in Chibok, Borno state, taking away the 219 others.
They described how they were held in eight different camps thought to be in the vast Sambisa Forest area of Borno and the Gwoza hills, which separates Nigeria from Cameroon.
Human Rights Watch said more than 500 women and girls have been abducted since the start of the insurgency in 2009, although other estimates put the figure in the high hundreds.
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