A key leader of the Vanguard for the Protection of Muslims in Black Lands (Ansaru) terror group has claimed his militants could kill Abubakar Shekau, the disputed leader of Boko Haram terrorists.

It is believed that Mamman Nur wrote an open letter to Shekau days after Boko Haram's ally, Isis, appointed the Nigerian terror group's former spokesperson Abu Musab Al-Barnawi as its new leader.

Shekau, however, denied he had been replaced and vowed to continue his fight.

Shekau has been Boko Haram's leader since 2009. However, since pledging allegiance to IS in 2015, he has been increasingly criticised for targeting Muslims during terror attacks and ignoring IS directives.

"He [Shekau] has killed and he will continue to kill because he has evidence for that, forgetting the fact that we are not afraid of death. Meanwhile, we are out because of it," Nur is believed to have said in the letter.

"He has forgotten that we just came out of his room. If we had wanted to kill him we would have done so, because the security [guards] surrounding him are with us and he does not know them, but if he touches us they will also touch him," continued Nur, referring to the fact Ansaru is a Boko Haram splinter.

Who is Mamman Nur?

Nur was born in Maroua, the capital of Cameroon's Far North Region. Little is known about him. It is believed he wielded considerable influence when he was a BokoHaram member.

Some analysts claim he acted as the third in command when the group was led by its founder Mohammed Yusuf, who was killed while in police custody in 2009 and replaced as leader by Shekau.

It is believed Nur introduced Shekau to Yusuf. Some analysts claimed Shekau was chosen as Yusuf's successor because Nur, unlike most Boko Haram commanders, does not come from the Kanuri tribe.

"Nur was one of Yusuf's few followers who fled to East Africa (reportedly to Somalia) and trained with al-Shabaab and Al-Qaeda in the Islamic Maghreb (Aqim) militants," a report by the Combating Terrorism Centre (CTC) said.

When Nur returned to Nigeria in 2011, the country's security services identified him as the mastermind behind a Boko Haram attack that killed 23 people in the capital Abuja.

Nur, Khalid Al-Barnawi – now in DSS (Department of State Services) custody – and other Boko Haram members defected to form Ansaru in 2012.

The splinter group operates in north-eastern Nigeria and, unlike Boko Haram, does not attack Muslim civilians or force women and children to carry out suicide bombing missions.

"Shekau ordered the 'death penalty' for anyone who did not follow his orders and killed defectors. This may have been the origin of the split between Boko Haram and Ansaru in 2012," CTC explained. "Shekau's reported favouritism of ethnic Kanuris of Borno also may have driven Hausas, non-Nigerians and other non-Kanuris to ally with Nur, who was also non-Nigerian."

After defecting, Nur became a key leader of Ansaru, which has links to both Aqim and al-Shabaab terror groups. Although Ansaru is a splinter of Boko Haram, it is believed the two groups reintegrated in the Nigerian-Cameroon border region, where they exchange resources and militants.

Nur could be behind split

Following Shekau's replacement, analysts warned the group had split into two factions, one loyal to Shekau and the other to Abu Musab Al-Barnawi. Some counter-extremism experts suggested the future development of Boko Haram could depend on Nur, given his connection to international terror groups.

"Nur seems to be the main instigator of the split," David Otto, CEO of global security provider TGS Intelligence Consultants, told IBTimes UK. "He seems to be the one pulling the strings behind. He could be using Abu Musab as a forefront, but the content of the letter shows he is in full control."

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Who are Nigeria's Boko Haram terrorists?

Boko Haram, which has renamed itself Iswap, fights against Western influence in Nigeria and aims to impose its version of Sharia law throughout occupied territories.

The group launches attacks in Nigeria and neighbouring countries in a bid to take control of more territory. Three Nigerian states − Adamawa, Borno and Yobe − have been under a state of emergency since May 2013.

Boko Haram has killed more than 20,000 people since 2009 and was deemed the world's deadliest terror group, surpassing Islamic State in November 2015. Nigeria has also become the world's third-most terrorised country as a result of the group's violent insurgency.