Nigeria's military claimed Tuesday to have seriously injured Boko Haram's elusive leader Abubakar Shekau and killed other commanders in an air strike on the Islamist group's forest stronghold.

The announcement was met with scepticism by Boko Haram experts who pointed out that Nigeria has repeatedly claimed to have killed Shekau in previous raids only for the militant leader to surface shortly afterwards in videos.

US Secretary of State John Kerry, who is on a visit to the northern Nigerian city of Sokoto, praised the country for its recent gains against the Islamic insurgents but made no mention of the claims that the group's leader was injured.

Shekau was "fatally wounded in the shoulder" during Friday's raids, said army spokesman Sani Usman in a statement, without giving further details.

"I said wounded, if there is any other development I will let you know," Usman told AFP in a text message.

Usman also said three Boko Haram commanders -- Abubakar Mubi, Malam Nuhu and Malam Hamman -- were confirmed dead with several others injured.

The claim comes as Kerry visits the country on a two-day trip for talks likely to focus on the fight against Boko Haram and the sputtering economy.

"It is unclear if there is any other confirmation that Shekau himself has died of his wounds, and we should remember that Shekau has been claimed dead on a number of occasions before," Omar Mahmood, a researcher at the Pretoria-based Institute for Security Studies, told AFP.

"Nonetheless, the announcement comes at an interesting time given the recent leadership rift within the movement and if Shekau is incapacitated as a result that will be a major blow to his faction."

The Nigerian military may be trying to prove the legitimacy of using airstrikes after a video was released earlier this month claiming that some of the 218 kidnapped Chibok girls had died following an aerial attack on Boko Haram camps by government forces.

"This is the Nigerian military saying that this is a very viable strategy," security analyst Ryan Cummings said.

The mysterious Shekau's fate has been the subject of speculation recently amid claims he had been replaced by Sheikh Abu Musab al-Barnawi, the group's former spokesman.

Barnawi's appointment was contained in a magazine issued by the Islamic State group, to which Boko Haram pledged allegiance in March last year.

But only a week later, the shadowy Shekau surfaced in a video posted on social media, ridiculing suggestions of his death and looking more composed and energetic than in previous appearances.

Boko Haram, which seeks to impose strict Islamic law in Nigeria's mainly-Muslim north, has killed some 20,000 people and forced at least 2.6 million others to flee their homes since 2009.