The leader of Boko Haram, the terrorist group which has caused the deaths of thousands of civilians across Nigeria, has been dead for some time, according to the Nigerian military.

A series of tweets from the Nigerian Army's Twitter account on Wednesday claimed that Abubakar Shekau is "deceased" and a lookalike, Mohammed Bashir, who has allegedly been posing as the Boko Haram leader in recent videos, has now also been killed.

This was confirmed by military spokesman General Chris Olukolade at a press conference in Abuja.

It is unclear where in Nigeria Bashir was when he was killed in an offensive against the terrorist group last week.

However, whether or not Abubakar Shekau has actually died remains a mystery.

Infamous for his extreme violence, Shekau has a $7 million bounty on his head after being labelled a "Specially Designated Global Terrorist" by the US Department of State in 2012. Shekau has repeatedly expressed an intent to cleanse Nigeria of all those who oppose his extreme Islamic ideology, stating in one video that Boko Haram will "soak the ground of Nigeria with Christian blood".

Police in Maidguri reported that he had been killed in July 2009, and in August 2013, the security taskforce reported that Shekau "may have died" from gunshot wounds during a clash on the Cameroon border two months earlier.

However, the high command in Abuja has never officially confirmed Shekau's death,and Boko Haram has since released several videos in which Shekau, or possibly a lookalike, declares that he remains alive and in charge of the militant group.

"Here is Shekau, Shekau, Shekau, Shekau, original. I want the whole world to know that I'm alive by the grace of Allah," the alleged leader said in a video released last year.

Adding to the confusion, the spokeswoman for the country's Department of State Services, Marilyn Ogar, told reporters in May 2014: "What I know is that the original Abubakar Shekau is dead.

"The person claiming to be the national leader (of Boko Haram) is not the original Abubakar Shekau," she added, according to Nigeria's 'The News'.

Military spokesman General Chris Olukolade told a news conference in Abuja yesterday that the name Shekau has become "a brand name for terrorists" and Bashir was merely "posing" as the original.

However, it is as yet unknown whether or not the real Shekau remains at large, and the BBC's Will Ross in Lagos says it is "impossible to verify" the latest claims.

According to John Campbell of the Council on Foreign Affairs, who "suspect[s]" Shekau is alive, however, the fate of this one man may be of little importance.

"The movement [of Boko Haram] is remarkably resilient, and not dependent upon a single leader...Boko Haram is more than Abubakar Shekau, alive or dead," he said.