Amnesty International said Wednesday there was sufficient evidence for the International Criminal Court to probe senior Nigerian military officers for war crimes in the battle against Boko Haram.

The group stated the case against five senior officers in a new 133-page report based on hundreds of interviews, including with military sources, and using leaked defence ministry documents.

The allegations centre on thousands of people Amnesty claimed were extra-judically murdered by the security forces and its civilian vigilante allies, as well as crimes against those held in military custody.

Amnesty's Secretary General Salil Shetty described the evidence of abuses collected as "sickening".

"In the course of security operations against Boko Haram in north-east Nigeria, Nigerian military forces have extrajudicially executed more than 1,200 people; they have arbitrarily arrested at least 20,000 people, mostly young men and boys," the report said.  

Nigerian forces "have committed countless acts of torture; hundreds, if not thousands, of Nigerians have become victims of enforced disappearance; and at least 7,000 people have died in military detention as a result of starvation, extreme overcrowding and denial of medical assistance", it added.

Amnesty said commanders based in the northeast "should be investigated for potential responsibility for war crimes of murder, enforced disappearances and torture".

Top service chiefs in the capital Abuja "should be investigated for their potential command responsibility for crimes committed by their subordinates given that they knew or should have known about the commission of the crimes, and failed to take adequate action".

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

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

But the report said: "Amnesty International believes that the evidence contained in this report and submitted separately to the (ICC) Office of the Prosecutor is sufficient to reopen this issue."

- Buhari urged to act -

The latest report includes new claims on specific acts of murder allegedly committed by Nigerian soldiers during the conflict but the general subject matter is not new.

Amnesty, Human Rights Watch and independent reporting, including by AFP, has found evidence of arbitrary killing, unlawful detention and torture in custody of alleged Boko Haram suspects.

Shetty stressed the report was not just about calling for the prosecution of individual officers.

It also aimed to highlight "the responsibility of Nigeria’s leadership to act decisively to end the pervasive culture of impunity within the armed forces", he said.

The government of former president Goodluck Jonathan was repeatedly pressured to try all Boko Haram suspects in court.

Charges have been filed against a select few individuals arrested in the northeast but arbitrary and seemingly indefinite detention has persisted.  

New President Muhammadu Buhari said after taking the oath of office on Friday he would review military rules of engagement to try to end concerns of rights violations by soldiers.

He also promised to improve "operational and legal mechanisms so that disciplinary steps are taken against proven human right violations by the Armed Forces".

Following Buhari's election victory over Jonathan in March, world powers, including the United States and Britain, offered Nigeria's new administration additional cooperation against Boko Haram.

Amnesty's senior director for research, Anna Neistat, told reporters in Abuja that such offers could be conditional on whether Nigeria supports a probe into the alleged military abuses.

"We insist that the International Criminal Court should consider the evidence contained in this report as part of its preliminary examination of the situation in Nigeria, to identify specifically what step Nigeria is taking to bring those responsible for war crimes documented in this report to justice," she said.

The ICC should also "reconsider its previous findings that there is insufficient evidence to conclude that the members of the Nigerian military and not only members of the Boko Haram have committed crimes against humanity."