A suicide bomber has attacked the main military base in the northern Nigerian city of Maiduguri, military sources have told the BBC. The attacker died and one soldier was injured in the explosion at the headquarters of the Joint Task Force. There have been three other explosions on Friday, one outside a college, the Borno state police chief told AP. The Islamist Boko Haram group has carried out several similar attacks in Maiduguri and other Nigerian cities. Boko Haram - which means \"Western education is forbidden\" - usually targets the police and government officials.