Hundreds of people have died in detention facilities in north-east Nigeria as the army tries to crush an Islamist militant rebellion there, according to Amnesty International.
The human rights group said some detainees died from suffocation in overcrowded cells, others from starvation and extra-judicial killings.
It is calling for an urgent investigation into the deaths.
There has not yet been an official response to the report.
But the Nigerian army has rejected all previous accusations of human rights abuses.
A senior Nigerian army officer told Amnesty that at least 950 people had died in military custody during the first half of this year.
Most had been accused of having links to the Islamist militant group Boko Haram, Amnesty said.
Boko Haram is fighting to overthrow Nigeria's government to create an Islamic state, and has launched a number of attacks on schools.
About 50 students were shot dead earlier this month in their hostel, in an attack blamed on Boko Haram.
A state of emergency was declared in three northern states in May – Yobe, Borno and Adamawa – in response to thousands of deaths in militant attacks.
But while most of the recent news from has been about these civilian killings, the BBC's Nigeria correspondent Will Ross says this latest Amnesty report shines a light on another grim side of life in northern Nigeria.
At times, the number of people killed in these detention centres was so high that there were regular mass burials, Amnesty said.
The BBC has seen photos of bodies reportedly dumped outside the mortuary in the city of Maiduguri by the military.
The bodies showed no obvious signs of having been killed in combat.
Amnesty has called for an urgent investigation, but those who follow events closely in Nigeria will know that such an investigation is highly unlikely to happen, a BBC correspondent says.