Boko Haram's long-time leader Abubakar Shekau has said in an audio message he is "still around" despite his reported removal as leader of the Nigeria-based armed group by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

ISIL had previously named Abu Musab al-Barnawi as Boko Haram's new leader, prompting the message from Shekau.

"People should know we are still around," Shekau said in the 10-minute message on Thursday.

In an interview published by the ISIL newspaper Al Nabaa on Wednesday, Barnawi, the ISIL appointee, threatened to bomb churches and kill Christians while ending attacks on mosques and markets used by Muslims. 

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"They strongly seek to Christianise the society. They exploit the condition of those who are displaced under the raging war, providing them with food and shelter and then Christianising their children," SITE Intelligence Group quotes the new leader as saying.

Barnawi said the fighters will respond by "booby-trapping and blowing up every church that we are able to reach, and killing all of those (Christians) who we find from the citizens of the cross".

The title of "wali", now given to the new leader, was previously used to describe Shekau.

The report did not say what Shekau's current status was, although there have been rumours for weeks that he had been replaced.

Al Jazeera's Ahmed Idris, reporting from Lagos, quoted security sources as saying that the announcement was a sign of desperation among Boko Haram fighters and an effort to reinvent themselves.

"Boko Haram has lost much of its territory to regional forces from Nigeria, Cameroon, Chad and the Niger Republic, so the security forces believe that this is a sign of Boko Haram trying to be relevant in the scheme of things," he said.

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The interview with Barnawi indicates a major shift in strategy for the fighters, who have killed many more Muslims than Christians in attacks in mosques with suicide bombers and armed men.

There have also been attacks on crowded marketplaces in predominantly Muslim areas and the killings and kidnappings of schoolchildren.

The targeting of students accounts for its nickname Boko Haram, which means Western education is sinful or forbidden.

In March 2015, Shekau switched allegiance from al-Qaeda and declared that Boko Haram be known as the ISIL's West Africa Province.

At the time, Boko Haram was the most powerful military force in northeast Nigeria, controlling a huge area and was better equipped and motivated than Nigerian forces.

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Under Shekau, the seven-year campaign spread to neighbouring countries, killed more than 20,000 people and drove more than 2.2 millon from their homes.

It created what aid workers are calling a catastrophic humanitarian emergency with children dying of starvation daily.

Boko Haram last week ambushed a humanitarian convoy, killing three civilians including a UN employee and causing the suspension of UN aid to newly liberated but still dangerous areas of Nigeria's northeast.

Source: Agencies