Hundreds of people are dying in military detention as Nigeria's security forces crack down on an Islamist uprising in the north-east, Amnesty International has warned.
Some people have been shot outright, some have starved and others have been suffocated to death, the rights watchdog said.
"Others were reportedly shot in the leg during interrogations, provided no medical care and left to bleed to death."
More than 950 people died in military custody in the first six months of this year, according to "credible information" from a senior Nigerian army officer.
If true, that would mean that Nigeria's military killed more civilians than Boko Haram during the first half of 2013.
Amnesty called for an urgent investigation.
Nigeria's security forces are notorious for extrajudicial killings.
Hundreds of detainees have gone missing since the government imposed a state of emergency on May 14 on three north-eastern states.
Witnesses said hundreds of people had been rounded up, often indiscriminately, in night raids.
Relatives, human rights organisations and journalists have asked the army, the police, intelligence services and government officials where the arrested people were, but answers have never been provided.
Nigerian Civil Rights Congress spokesman Shehu Sani said he believes thousands are now detained.
Amnesty said those killed were first detained as suspected members or associates of Boko Haram, which has claimed responsibility for attacks that have killed hundreds of civilians this year.
Most deaths recorded by Amnesty took place at the presidential lodge guardroom, a detention centre in Damaturu or at Giwa military barracks in Maiduguri, the capital of Borno state and the birthplace of Boko Haram.
"Hundreds have been killed in detention either by shooting them or by suffocation. People are crammed into one cell," Amnesty quoted a senior army officer as saying.
"There are times when people are brought out on a daily basis and killed."
Amnesty deputy Africa director Lucy Freeman said: "The details of what happens behind locked doors in these shadowy detention facilities must be exposed and those responsible for human rights violations brought to book."
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