Nigeria will investigating allegations some 8,000 people detained during a crackdown against Islamist militant group Boko Haram were killed, weeks after dismissing a report by Amnesty International.
Army leaders initially rejected the allegations of prisoners being executed and mistreated, published earlier this month, as "biased and concocted".
Amnesty said Nigerian troops had rounded up thousands of men and boys, some as young as nine, in Boko Haram strongholds.
The report said that many some prisoners had died due to starvation, overcrowding, torture and denial of medical care.
But international pressure has been mounting on Nigeria to examine its tactics, and soon after the report's release, recently-elected President Muhammadu Buhari promised his office would study it and "act accordingly".
The armed forces called a press conference on Friday to say investigations had started.
"The military has a constitutional and moral responsibility to protect Nigerian citizens and cannot suddenly engage in mass murder as portrayed by Amnesty International allegations," said Major General Adamu Baba Abubakar.
He said Amnesty had not accepted an offer from the military to provide a representative to sit on the investigation panel - an offer he said was made to ensure fairness and show "the military has nothing to hide".
Boko Haram has killed thousands and forced about 1.5 million people to flee in a six-year battle to set up an Islamic state in the remote northeast of the country.
The military initially struggled to contain the militants and their guerilla-style attacks and kidnappings.
But Nigeria has recently had more success in pushing Boko Haram back, with the help of troops from neighbouring Chad, Niger and Cameroon.