Global rights watchdog Amnesty International accused Nigeria's military and its supporting civilian militia on Tuesday of "extensive human rights violations" in their fight against Boko Haram in the country's northeast.

Amnesty said gruesome video footage, images and witness testimonies gathered during a recent research mission to Borno State provided "fresh evidence of extrajudicial executions and serious human rights violations" carried out in the region as Nigerian troops battle the extremist group.

The footage includes images of detainees having their throats slit one by one and dumped in mass graves "by men who appear to be members of the Nigerian military and the Civilian Joint Task Force (CJTF), state-sponsored militias," it said.

"The ghastly images are backed up by the numerous testimonies we have gathered which suggest that extrajudicial executions are, in fact, regularly carried out by the Nigerian military and CJTF," said Salil Shetty, Amnesty's secretary general. "These are not the images we expect from a government which sees itself as having a leadership role in Africa."

The video also shows the aftermath of a Boko Haram raid on a village in which the militant group killed nearly 100 people and destroyed scores of houses and buildings, Amnesty said.

"More than 4,000 people have been killed this year alone in the conflict (between) the Nigerian military and Boko Haram, including more than 600 extrajudicially executed," it added

Boko Haram abducted more than 200 schoolgirls from their dormitory in April in northeastern Nigeria. The militants are still holding 219 girls captive.

The kidnappings prompted a social media campaign that went viral and drew unprecedented global attention to Boko Haram, which aims to create a strict Islamic state in Nigeria's mainly Muslim north.

Attacks had been concentrated in the remote northeast, Boko Haram's stronghold, but a spike in violence since April in major cities including Abuja has shown the grave threat the Islamist group poses to Nigeria, Africa's most populous country and its top oil producer.

Boko Haram staged its most deadly assault on Bama town in February, which locals said left almost 100 people dead and more than 200 injured. Improvised explosive devices and grenades were used to destroy swathes of the town.

Amnesty called on the Nigerian authorities to ensure that the military stops committing violations of human rights and urged detailed investigations into the most serious alleged cases.

- 'Very serious' -

Nigeria's defence ministry responded swiftly to the claims, saying it took the allegations "very seriously".

The military high command established a team of senior officers, legal and forensic experts to study Amnesty's footage and "allegations of infractions in order to ascertain the veracity of the claims with a view to identifying those behind the acts".

"Much as the scenes depicted in these videos are alien to our operations and doctrines, (they have) to be investigated to ensure that such practices have not crept surreptitiously into the system," an army statement said.

Amnesty and local rights groups often accuse the military and police of extrajudicial killings and human rights violations. Officials deny the charges.

The police said recently that it was introducing human rights studies into its training courses.