A loud explosion has rocked the centre of the north-eastern Nigerian city of Maiduguri - a stronghold of the Islamist Boko Haram group. The BBC's Bilkisu Babangida in the city says attacks by militants are common but such a loud blast is unusual. According to Reuters news agency, the blast shattered windows in surrounding neighbourhoods. Boko Haram says it is fighting for Islamic rule, and campaigns against all activity associated with the West. At least 40 people have died in Maiduguri in the past 10 days in attacks blamed on the sect. Vow of vengeance Our correspondent says the blast occurred just after 0700 local time (0600 GMT) in the Abba Ganaram area, which has experienced many attacks blamed on Boko Haram. "People ran out to see where this thing had happened. There were huge plumes of smoke," one witness told Reuters. Soldiers have blocked some roads into the centre of the town, the agency reports. The group's trademark has been the use of gunmen on motorbikes. Two years ago, Nigerian security forces brutally suppressed an uprising by Boko Haram, destroying their compound and then killing their leader in custody. Instead of disappearing, the group has regrouped and vowed to revenge their leader's death. Dozens of people have been killed - mostly security officers and politicians but also a Christian preacher and Muslim clerics who have criticised Boko Haram. The group also attacked the police headquarters in the capital Abuja earlier this month. Source: BBC