Nigeria's military has killed a top commander of militant Islamist group Boko Haram in the north-eastern city of Maiduguri, an army spokesman has said, according to BBC News.

Ibn Saleh Ibrahim was killed in an exchange of fire with six of his lieutenants, the spokesman added.

An unknown number of civilians are said to have been killed in the crossfire.

Boko Haram, which has killed hundreds of people since 2009, has not commented on Mr Ibrahim's reported death.

The group's founding leader, Mohammed Yusuf, was killed by security forces in July 2009.

Earlier this month, rights group Amnesty International accused Nigeria's security forces of carrying out widespread abuses in their campaign against Boko Haram, including extrajudicial killings, enforced disappearances and torture.

The group is campaigning to impose Islamic law across Nigeria.