A suicide bomber has attacked the main military base in the northern-eastern 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.The sect is thought to have carried out the bombing in August of the UN headquarters in the capital, Abuja, killing 23 people.A military source told the BBC that another of Friday's attacks was at the Maiduguri headquarters of the State Security Service.Military spokesman Lt Col Hassan Ifijeh Mohammed told the Associated Press news agency that a four-wheel-drive vehicle tried to enter the army base and exploded after it was refused entry.Friday's attacks follow a special operation by the security services in the city to mop up illegal weapons.There have been house-to-house searches this week looking for arms after the deadline for a gun amnesty expired on 31 October.Meanwhile riots have broken out in Zonkwa, a town in the north-central state of Kaduna, after a church was attacked by unknown gunmen.Hundreds of people were killed in Kaduna after April's election and some of those attacked had vowed reprisals, as had Boko Haram.