More than 8,000 people have died while being detained by Nigeria's armed forces during the campaign against militant Islamist group Boko Haram, Amnesty International said on Wednesday, allegations that the military denied.

The group said many of the prisoners were executed and others died due to starvation, overcrowding, torture and denial of medical assistance.

Boko Haram's six-year insurgency has killed thousands and left 1.5 million people displaced. The group wants to establish an Islamic caliphate in the northeast of Nigeria, Africa's biggest economy and top oil exporter.

Muhammadu Buhari, the new president, has vowed to defeat Boko Haram and was holding talks on Wednesday with his counterparts in neighboring 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 Niger, Chad and Cameroon.

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.

More than 1,200 people were extrajudicially executed and more than 7,000 starved or died of disease in severely overcrowded cells, Amnesty said.

Many of the people executed were shot dead inside detention facilities, despite presenting no danger, in violation of international humanitarian laws, Amnesty said.

"BLACKMAIL"

"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.

Major General Chris Olukolade said the charity was trying to "blackmail" the country's armed forces and no allegations had been proved against individuals who the report identified.

"The Nigerian military ... rejects the biased and concocted report provided by Amnesty International," he said in a statement. "The Nigerian military does not encourage or condone abuse of human rights, neither will any proven case be left unpunished."

Amnesty's 133-page report was based on about 400 interviews with sources, including victims, eyewitnesses and members of the armed forces, as well as videos and photographs.

Presidency spokesman Garba Shehu said Buhari's administration would study the report and act appropriately.

"The administration will leave no stone unturned to promote the rule of law. ... Respect for human rights and adherence to the rule of law are the life and soul of the democratic system," Buhari was quoted as saying in a statement from his spokesman.

The British High Commission in Abuja said it regularly raises human rights issues with Nigeria but was encouraged by Buhari's inaugural speech that reforms would be made to improve the military's behavior and strengthen sanctions on such abuses.

A US State Department official said that Nigeria should take the report seriously and investigate the allegations.

"We have learned ourselves, fighting a brutal terrorist organization while maintaining a high standard of professional conduct...can be a challenge, one that we remain committed to helping our Nigerian partners to meet," the official said.