The son of Nigeria's former President Olusegun Obasanjo has been shot and wounded in a battle with militant Islamists, the ex-leader's aide says.

Lt Col Adeboye Obasanjo was injured as the army fought to recapture towns in north-eastern Adamawa state from Boko Haram, Muhammad Keffi told the BBC.

"Scores of insurgents" were also killed in the battle, the military said.

Last month, Boko Haram declared an Islamic state in areas it controls in north-eastern Nigeria.

Experts raised concerns that Nigeria, Africa's most populous country, could break up in a way similar to Iraq and Syria where the militant group Islamic State (IS) has declared a caliphate.

Boko Haram insurgents have over-run Michika and Bazza, two towns close to the commercial hub of Mubi, in recent days.