Amnesty International said Wednesday there was sufficient evidence for the International Criminal Court to probe senior Nigerian officers for war crimes in the battle against Boko Haram, prompting an angry rejection by the country's military.

In a 133-page report, the watchdog claimed that hundreds of people had been unlawfully killed and thousands more had died in military custody in recent years, including as a result of torture and starvation.

Nigeria's new president Muhammadu Buhari promised to look into the allegations, which target five senior officers and are based on leaked defence ministry documents and hundreds of interviews.

"This administration will leave no stone unturned to promote the rule of law and deal with all cases of human rights abuses," Buhari was quoted as saying in a statement.

Amnesty's secretary general Salil Shetty described the abuses as "sickening" but Nigeria's military said the report was designed to besmirch its reputation.

"In the course of security operations against Boko Haram in northeast Nigeria, Nigerian military forces have extra-judicially 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.

- War crimes -

London-based Amnesty said commanders in Nigeria's 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.

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 Nigerian leader Buhari has made the fight against Boko Haram a priority and said after taking the oath of office on Friday that 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".

- Military hits back -

Nigeria's military accused Amnesty of peddling an agenda against it, publishing a "biased and concocted report" to discredit the country and its efforts to defeat extremism.

"The officers mentioned in the report have no reason whatsoever to indulge in the allegation made against them," defence spokesman Chris Olukolade said in an emailed statement.

"It is unfortunate that the organisation just went out to gather names of specified senior officers, in a calculated attempt to rubbish their reputation as well as the image of the military."

Olukolade rejected claims the military and government had failed to respond to previous allegations of abuses and said detention facilities had been opened up to rights bodies and other groups.

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

But "so far no allegation has been sufficiently proved against those whom Amnesty International is so desperate to convict, Olukolade added.