More than 8,000 people have died while being held prisoner by Nigeria's armed forces in the campaign against Islamist group Boko Haram, many of them deliberately killed, Amnesty International said on Wednesday.
In a 133-page report, the group said more than 1,200 people had been extrajudicially executed since March 2011 and over 7,000 had died in a military detention due to starvation, overcrowding, torture and denial of medical assistance.
A spokesman for Nigeria's armed forces was not available for comment. A presidential spokesman who spoke to Reuters by phone had no immediate response to the report.
Boko Haram has carried out a six-year insurgency during which it has tried to establish an Islamic caliphate in the northeast of Africa's biggest economy and top oil exporter, killing thousands and leaving 1.5 million people displaced.
Muhammadu Buhari, the new president, has vowed to defeat Boko Haram, calling the group "mindless" and "godless", and was holding talks on Wednesday with his counterparts in Niger and Chad on how best to tackle the insurgency.
The militants controlled a swathe of territory around the size of Belgium at the start of the year but have lost most of it in recent months due to the combined efforts of troops from Nigeria and neighbouring Niger, Chad and Cameroon.
Amnesty International said many of the people executed by the armed forces had been shot dead inside detention facilities, despite presenting no danger, in violation of international humanitarian laws.
In his inauguration speech last week, Buhari said allegations of human rights abuses, which have dogged the armed forces in the last few years, would be addressed. "We shall improve operational and legal mechanisms so that disciplinary steps are taken against proven human rights violations by the armed forces," he said.
Amnesty said Nigerian troops had rounded up thousands of men and boys, some as young as nine, in Boko Haram strongholds. Many of those held were executed or died in custody if their families were unable to pay a bribe. Lack of food caused many to starve, and severe overcrowding led to the spread of diseases.
"These acts, committed in the context of a non-international armed conflict, constitute war crimes," said Amnesty, adding that senior military commanders should be investigated for possible crimes against humanity. "Evidence shows that senior military leaders knew, or should have known, about the nature and scale of the crimes being committed," it said.
Amnesty said its report was based on about 400 interviews with sources including victims, eyewitnesses and members of the armed forces, as well as videos and photographs.