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.Ibn Saleh Ibrahim was killed in an exchange of fire with six of his "lieutenants", the spokesman added.Nigeria's security forces believe he was behind last month's assassination of retired General Mohammed Shuwa.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.'War hero'Boko Haram is now said to be led by Abubakar Shekau.Army spokesman Lt Col Sagir Musa told the BBC that Mr Ibrahim was "very close" to Mr Shekau and had a reputation of being "invincible"."Yesterday [Thursday], we learned he was in town and we were able to track him in a special operation," he said."There was an exchange of fire and in the process he was killed with six of his lieutenants."In a statement, Lt Col Musa said the operation in Maiduguri, which supported by armoured personnel carriers and helicopters, was on-going.Weapons and explosive devices have been recovered, he added.Lt Col Musa said Mr Ibrahim had had been responsible for the assassination of Gen Shuwa following an order from Mr Shekau.Gen Shuwa is regarded by the Nigerian military as a war hero, and played a key role in crushing Biafran separatists during Nigeria's brutal civil war in the 1960s.He was shot by gunmen in his home in Maiduguri, but no group has said it carried out the killing.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.