Lagos, Nigeria - The Nigerian military, including nine senior commanders, should be investigated for their role in shooting, starving, suffocating and torturing thousands of people to death during the fight against Boko Haram, Amnesty International says.

More than 7,000 boys and young men have died under military detention since March 2011, the UK-based rights organisation says in a new report, Stars on Their Shoulders. Blood on their Hands.

A further 1,200 have been rounded up and extrajudicially killed since February 2012, it says.

A senior military figure told Amnesty that it was common for soldiers to "go to the nearest place and kill all the youths. People killed may be innocent and not armed".

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At least 20,000 young males, some only nine years old, have been arrested since 2009, when Boko Haram, which opposes Western influence, began their armed campaign to form a state ruled by Islamic law.

In most cases, the arrests have been illegal.

Detainees have been kept in cramped cells with little food or water, without investigation or trial, Amnesty says.

"This sickening evidence exposes how thousands of young men and boys have been arbitrarily arrested and deliberately killed or left to die in detention in the most horrific conditions," Salil Shetty, Amnesty International's secretary-general, said.

The report, which drew on leaked military reports as well as interviews with victims and security forces, states that senior commanders either sanctioned the abuses, or ignored them when they took place.

Nine senior officers should be urgently probed for individual or command responsibilities for the human rights abuses, Amnesty says.

Those include General Ken Minimah, the incumbent army chief of staff; Air Chief Marshal Badeh, chief of defence staff; and both of their predecessors.

Military rejects claims

Responding to the report, Major General Chris Olukolade of Nigeria's military said Amnesty International was trying to "blackmail" the armed forces and that no allegations had been proved against individuals the report identified.

"The Nigerian military ... rejects the biased and concocted report," he said in a statement on Wednesday.

"The Nigerian military does not encourage or condone abuse of human rights, neither will any proven case be left unpunished."

Reports suggest Nigeria's security forces grew increasingly heavy-handed as momentum turned against them in the fight against Boko Haram.

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At the height of their strength in January, the fighters were controlling a large expanse of northeastern Nigeria.

Multiple allegations of human-right abuses have been levelled against the military, prompting Western governments to cut arms sales to the country.

In his inauguration speech last Friday, President Muhammadu Buhari promised to overhaul the rules of engagement to avoid human rights violations by security forces.

"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's revelations will more than likely serve as a litmus test of President Buhari's commitment to respond to the ill-discipline of Nigeria's armed forces," said Ryan Cummings, chief security analyst for Africa at the South Africa-based consultancy Red24.

Anna Niestat, Amnesty's senior director of research, told Al Jazeera: "We are not only calling for individuals to be investigated.

"Safeguards must be put in place to combat this endless cycle of impunity and abuses."

Garba Shehu, a spokesperson for President Buhari, said his attention would be drawn to the report when he emerges from meetings in Niamey, Niger, on Wednesday.

Source: Al Jazeera