Boko Haram crisis: Nigerian army accused of 7,000 deaths
More than 7,000 men and boys have died in Nigerian military custody during its fight against Boko Haram over the last four years, Amnesty International says.
They are among more than 20,000 people who have been arrested during operations against the Islamist militants, the rights group says.
The military has rejected the allegations, calling the report biased and the statistics "spurious".
However, President Muhammadu Buhari said it would be investigated.
At least 17,000 people have died in the conflict since 2009, says Amnesty.
That means around 40% of all deaths have been in military custody.
About 1.5 million people have also been displaced and hundreds more abducted since Boko Haram launched its violent uprising to impose Islamic rule in 2009.
The report comes as Nigeria's new President Buhari makes his first foreign trip since taking office - to Niger - to discuss regional operations against Boko Haram.
BBC Nigeria correspondent Will Ross says Amnesty International and other human rights groups have accused Nigeria's security forces of carrying out many atrocities before.
But this report goes further as the UK-based rights group names several senior officers - including major generals and brigadier generals - and calls on them to be investigated for murder, torture and enforced disappearance, he says.
In response, military spokesman Maj Gen Chris Olukolade said the report "went out to gather names of specified senior officers, in a calculated attempt to rubbish their reputation".
Mr Buhari, however, said his administration would study the document and act appropriately.
"Respect for human rights and adherence to the rule of law are the life and soul of the democratic system. We will not tolerate or condone impunity and reckless disregard for human rights," he said while in Niger.
The report, entitled Stars on their shoulders, Blood on their hands, says the senior officers should either be investigated for carrying out the war crimes themselves or for being in command of subordinates who did so.
Amnesty International says more than 1,000 people have been unlawfully killed.
It says in some cases captives were deliberately starved in custody and boys as young as nine years old have been detained.
The human rights group calls on President Buhari to end the culture of impunity in the armed forces.
At his inauguration last week, Mr Buhari promised to "overhaul the rules of engagement to avoid human rights violations in operations".
This year Nigeria's army - backed by regional forces - has recaptured many towns and villages from the militants.
But the group is still holding many women, girls and children captive, including 219 schools girls it kidnapped from a school in Chibok in April last year.