Rights group Amnesty International called for an urgent investigation after saying it had evidence that hundreds of people had died in detention facilities during a crackdown on Islamists in Nigeria.
The group on Monday said prisoners had been suffocated, starved and subjected to extra-judicial killings in the army-led operation in the country's north east.
A senior army officer told the NGO that around 950 people suspected of having links to militant group Boko Haram had died in custody during the first half of the year.
The Islamist group wants to topple the Nigerian government and impose an Islamic state, and has been responsible for thousands of deaths.
The Nigerian army did formally respond, but has always rejected accusations of human rights abuses.
In its report, Amnesty called for an immediate probe but experts warned that their demands are likely to go unheeded.