Boko Haram's appointment of a new leader and his apparent rejection by the group's figurehead exposes wide divisions among the Nigerian jihadists as they come under pressure from the country's military.

The promotion of Abu Musab al-Barnawi, who is thought to favour more targeted attacks than the wholesale violence of figurehead Abubakar Shekau, was announced in the weekly magazine of Islamic State. Boko Haram, which has regularly staged suicide bombings in crowded areas, pledged loyalty to IS last year.

But Shekau appears to have rejected the new role of al-Barnawi, who experts say has been the group's military commander and has also been on the radar for months as head of a faction favoring attacks on the Nigerian military.

In a 10-minute audio clip on social media, a person purporting to be Shekau laid bare internal divisions by criticizing al-Barnawi's reported view that Muslims can live among non-Muslims without taking up arms.

"I am against the principle where someone will dwell in the society with the infidels without making public his opposition or anger against infidels," Shekau said in the local Hausa language.

"Anyone doing such can't be a real Muslim thick and thin."

Al-Barnawi's splinter group is based northeast of Maiduguri, the provincial capital of northeast Nigeria's Borno state and the epicenter of Boko Haram's seven-year armed attempt to create a regional Islamic caliphate, one Western security source said.

From there, it has been better-placed geographically to cultivate links across the Sahara with the Libyan arm of IS although the extent of direct practical ties between the two groups is not clear. Many experts say that the links are largely symbolic.

In the past, Shekau has appeared in propaganda videos draped in ammunition belts and brandishing automatic weapons as he spouted vitriol against Nigeria and the West.

His absence from any such videos since March has sparked speculation he has been wounded, is seriously ill or may even have died.

Limiting himself to an audio clip is likely to fuel suspicions of Shekau's reduced physical or operational capacity from the wilds of the northeast's Sambisa forest, where he has been penned in by the Nigerian army.

Nigerian President Muhammadu Buhari, a former military ruler, took office last year on a campaign platform to wipe out the group.

"Shekau and his clique remain in Sambisa, where they are under a lot of pressure from the Nigerian military push against Boko Haram," the security source said. "It's not existential yet but they haven't got as much freedom of movement as they did."

"FADING GROUP"

Boko Haram, whose name loosely means 'Western education is sinful' in Hausa, has suffered a dramatic decline in fortunes from 18 months ago when it controlled an area the size of Belgium and had Nigeria's military on the back foot.

Under Shekau, Boko Haram killed more than 15,000 people, displaced more than two million and attained worldwide infamy with the kidnapping of more than 200 schoolgirls from the town of Chibok in April 2014.

However since early 2015 it has suffered numerous defeats at the hands of the Nigerian, Cameroonian, Nigerien and Chadian militaries acting either individually or as part of a coordinated regional force.

Nigerian military spokesman Rabe Abubakar said the latest Boko Haram's leadership revelations were of "no relevance".

"We are just focused on clearing the remnants of the insurgents that are scattered around," he said. The leadership struggles were "the antics of a fading group", he added.

In the interview with IS's al-Naba magazine, al-Barnawi said Boko Haram was "still a force to be reckoned with" and that it had been receiving new recruits.

The splinter group's modus operandi appears to be targeting the Nigerian military rather than civilians.

That is in contrast to the bomb attacks on busy public places such as markets, mosques and camps for internally displaced people (IDP) that have typified its approach in northeast Nigeria, Niger and Cameroon over the last year.

"Under the Shekau reasoning, it is permissible to kill IDPs in camps," said Fulan Nasrullah, a security analyst based in northern Nigeria. Such action was not sanctioned by the IS-approved faction, he added.

But despite its set-backs, Shekau's supporters remained the largest part of the group, Nasrullah said.