The Nigerian military has dubbed an audio message released by terror group Boko Haram as "irrelevant and fruitless" days after the group said the army's claims of successful operations are lies.

The person heard speaking in the audio message was believed to be Abubakar Shekau, the Boko Haram's leader, who is rumoured to have been substituted after he failed to appear in the group's propaganda videos earlier this year. Shekau said in the message: "[The military] have lied about us, saying that they have retaken our territories, taken weapons and driven us away. They are actually the ones whom we have driven away. They are all liars."

However, Colonel Rabe Abubakar, defence spokesperson, urged Shekau, or whoever is acting as the the group's leader, to step down. He also alleged that some Boko Haram members have been surrendering to the army. "Our candid advice to Shekau or his ghost, cohorts or impersonators is to tow the line of their fellow comrades and surrender now as there is no more hiding place for him or his criminal gang to operate freely," Abubakar said in a statement.

He continued: "It is also necessary to tell those that are still masquerading as leadership of Boko Haram in whatsoever guise to quickly come to terms with the fact that era of cheap propaganda is over and in no distance time Nigerians and the whole world will know who is saying the truth."

Abubakar concluded saying that the military is working "round the clock" to defeat terrorism and called on Nigerian citizens to report suspicious behaviours. The army's statement was released days after nearly 100 people were killed and 90 injured in three attacks carried out in Maiduguri, capital of the restive Borno state, Boko Haram's birthplace.

Borno is the epicentre of Boko Haram's insurgency, which has resulted in the death of more than 15,000 people since 2009. The group, which aims to overthrow the current regime and establish an Islamic caliphate throughout the occupied territories, carries out attacks primarily in northern Nigeria and neighbouring Cameroon and occasionally in Niger and central Nigeria.

Since the appointment of President Muhammadu Buhari in May, attacks have increased in Nigeria's north-east and latest estimates suggest that at least 1.4 million children have fled their homes in the northeast as a result of the deadly insurgence. Nigeria is leading a new task force – consisting of 8,700 troops from Nigeria, Niger, Chad, Cameroon and Benin – which is believed to have scored some successes, such as the recapture of a key town in Borno.

Who are Nigeria's Boko Haram terrorists?

Boko Haram (recently renamed Iswap) fights against Western influence in Nigeria and aims to impose its version of Sharia law in the country.

The group declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.

Boko Haram has raided several cities in the north of the country in a bid to take control of more territory.

Three states − Adamawa, Borno and Yobe − have been under a state of emergency since May 2013, due to Boko Haram's attacks.

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