A video grab from Boko Haram terrorist network, shows the alleged missing girls abducted from the Chibok town in Nigeria. Photo: AP

A video grab from Boko Haram terrorist network, shows the alleged missing girls abducted from the Chibok town in Nigeria. Photo: AP A video grab from Boko Haram terrorist network, shows the alleged missing girls abducted from the Chibok town in Nigeria. Photo: AP

Nigeria's Boko Haram leader Abubakar Shekau, or a man claiming to be him, has been killed at least three times so far, according to the military, yet each time he apparently returns in the group's numerous jihadist videos.

Dead or alive, he appears to be fuelling violence which rights groups say is killing more people than at any time during Boko Haram's five-year-old reign of terror in the north of the 175 million-strong state, a Reuters report stated.

Officials say Shekau may be a name adopted by leaders of various wings of Boko Haram, raising the possibility the death of one may make others more amenable to negotiating an end to the fighting and release of 200 schoolgirls whose kidnap in April caused an international outcry.

The last time the military said he was dead a year ago, a man looking similar to Shekau but slightly fatter continued to appear in videos issuing threats and taunting authorities.

The Islamist insurgents have killed thousands of people, many of them civilians, since launching an uprising in 2009, and abducted hundreds of children in a tactic reminiscent of Ugandan rebel Joseph Kony's Lord's Resistance Army in central Africa.

Shekau's face has often appeared on video claiming attacks.

Now Nigeria's military says this video imposter -- real name Bashir Mohammed -- has died in fighting in the town of Kondugu, prompting the question whether there is another Shekau lookalike ready to continue the fight in Africa's top economy.

An alternative possibility is that Shekau is not dead.

"The Nigerian army has claimed on multiple occasions to have killed Shekau and it's been disproved," said Ben Payton, senior Africa analyst at UK-based risk consultancy Maplecroft.

Shekau took over when Boko Haram's founder and spiritual leader Mohammed Yusuf was killed in police custody in 2009.

It is not possible to verify whether or not the videos are of the same person or more than one, but the man in later videos that the military said was Bashir Mohammed had a plumper, rounder face, his nose was wider and his bridge less defined.

His talk was even more bellicose and maniacal than the original Shekau, with statements vowing to kill all pagans and saying he was against the ideology of the whole world.

The old Shekau was often quite sombre; the new one has a menacing laugh. And he only appeared after the alleged death of the real Shekau in August last year.

"The name Shekau has become a brand name for the terrorists' leader," military spokesman Major-General Chris Olukolade said on Thursday, an acknowledgment that however many Shekaus they kill, the violence is unlikely to end.

By contrast, the death of Angolan rebel leader Jonas Savimbi in battle in 2002 ended a quarter century of civil war in Angola.

"Even if Shekau has been killed ... Boko Haram is much bigger than one individual. It has multiple units that operate with a fair degree of autonomy," said Payton.

But if it is true that Nigerian forces have inflicted heavy casualties in the past few days on a faction of the militants, as Boko Haram sought to hold territory it declared to be an "Islamic state" two months back, the others might turn out to be slightly more moderate.

The military said on Wednesday more than 130 Boko Haram Islamist fighters had surrendered, and a man posing as the group's leader in numerous videos had been killed in clashes, although it often claims successes that are impossible to independently verify.

"It's very likely that there's two or three Shekaus and the commanders of different factions decided to all use the name," said Jacob Zenn, Boko Haram expert at the Jamestown Foundation.

"Possibly, that leaves ... an opportunity to capitalise on the death of this Shekau and start talks with various (other) members of Boko Haram."