A man claiming to be Boko Haram leader Abubakar Shekau appeared in a new video on Thursday, contradicting Nigerian military reports that he had been killed.

The video, seen by Reuters, shows a figure in boots and combat fatigues who looks similar to the man claiming to be Shekau in previous videos.

"Here I am, alive and I will remain alive until the day Allah takes my breath," he says in the northern Hausa language.

"Even if you kill me ... it will not stop us imposing Islamic rule ... We are still in our Islamic state, reigning and teaching the Koran."

Nigeria's military said last week that the man who had been posing as Shekau in the group's growing number of videos had been killed in clashes over the town of Konduga.

The military announced the death of Shekau a year ago, saying he had died in battle.

After that, the man appearing in videos as Shekau did look different than in previous images, with a wider nose, less defined bridge and a rounder face.

In the most recent video, this Shekau says "nothing will kill me until my days are over ... Some people asked you if Shekau has two lives. No, I have one life."

In a statement on its website, Nigeria's Defense Headquarters said the video did not indicate when it was shot with a screen time or date or provide any proof that Shekau was alive.

"As far as we are concerned, the individual who was appearing in video and claiming to be the leader of the terrorist group was killed in the Kondunga battle in September.

"The resemblance of the corpse and that of the eccentric character was incontrovertible. His identity was equally corroborated by people who knew him before we announced his death."

CAMPAIGN FOR ISLAMIC STATE

Boko Haram, whose violent five-year campaign for an Islamic state has killed thousands, has in the past two months progressed from bombings, raids and kidnappings to trying to seize territory in remote areas near theCameroon border, possibly inspired by similar moves by Sunni Islamist militants in Iraq and Syria.

The military has had mixed results trying to push back the militants, and low morale, a lack of discipline and poor equipment have hurt its ability to fight effectively.

Nigeria's military court martialled 97 of its troops for various offenses on Thursday, including mutiny, assault, absconding, house breaking and disorderly behavior, it said.

But holding territory has also made Boko Haram more vulnerable to attacks by Nigerian forces backed by fighter jets, security sources say. The military said it inflicted heavy losses on the Islamists in the past two weeks.

In the latest video, Shekau, bellicose as ever, stands on the back of a pick-up truck, firing an anti-aircraft gun into the air. The video also shows people being stoned to death and being given lashes, both traditional Islamic punishments.

There is also footage of Boko Haram fighters picking through the wreckage of an aircraft Shekau claims they shot down, and of a man they claim was its pilot with a noose around his neck. Nigeria's military is still looking for a war plane that went missing more than two weeks ago.

"We have shot down more than 10 of your planes," Shekau says. "We are the ones that killed your pilot, we slaughtered him. And there is nobody that we will not slaughter."