Amnesty International on Monday slammed the reinstatement of a senior Nigerian army officer it wants probed for war crimes and the mass murder of detainees in the fight against Boko Haram.

The rights group in June said Major General Ahmadu Mohammed along with eight other senior commanders should be investigated into their possible criminal responsibility for war crimes, including the deaths of more than 8,000 detainees.

It said there was sufficient evidence for the International Criminal Court, based in The Hague, to take up the case.

Amnesty said Mohammed was in charge of 7 Division of the army when the military allegedly executed more than 640 detainees after Boko Haram Islamists attacked the detention centre in Giwa barracks, Maiduguri, capital of the restive northeastern state of Borno on March 14 2014.

The London-based body said Mohammed was retired in 2014 for unrelated reasons, but that it received news of his reinstatement on January 17.

"Major General Mohammed must be investigated for participating in, sanctioning or failing to prevent the deaths of hundreds of people," said Salil Shetty, Amnesty's secretary general in a statement on Monday.

"Young men and boys, rounded up by the military, were either shot, starved, suffocated or tortured to death and no one has yet been held to account.

"It is unthinkable that Major General Mohammed could resume command of troops before an investigation has even begun."

"Seven months after the publication of these horrific discoveries and the (Nigerian) President's pledge that they will be looked into, we continue to call for urgent independent investigations to begin," said Shetty.

- 'Justice for the dead' -

"Only then can there be justice for the dead and their relatives."

Nigerian military authorities on Monday confirmed Mohammed had been recalled.

"The allegation against Brigadier General Mohammed remains an allegation until proven," defence spokesman Rabe Abubakar told AFP.

"As far as the military authorities are concerned, he is innocent and should not be punished uneccessarily," he said.

He accused Amnesty of shunning a board of inquiry it set up to look into various allegations of abuses by its troops in the war against Boko Haram insurgents.

"We cannot just punish men and officers who are sacrificing their lives to defend their fatherland against the terrorists just because Amnesty is saying so," he said.

Amnesty in an in-depth report had exposed a range of war crimes and possible crimes against humanity committed by the military in the course of its operations against Boko Haram.

It found that, since March 2011, more than 7,000 were starved, suffocated, and tortured to death in military detention camps.

A further 1,200 were said to have been rounded up and unlawfully killed.

The group said since the publication of the report, four of the named military commanders had retired.

Two others had already retired prior to the publication of the report, with the current status of two Brigadier Generals unknown, it said.

The ICC in The Hague has opened a preliminary investigation into the Boko Haram conflict, which Amnesty said has killed at least 17,000 people since 2009.

The tribunal has previously said there was insufficient evidence tying Nigeria's military to systematic and orchestrated atrocities targeting civilians.