A blast has rocked an area near a mosque in the restive north-eastern Nigerian city of Maiduguri, with witnesses reporting four deaths.In a BBC interview, Nigerian army spokesman Brig Gen Isa blamed the Islamist group Boko Haram.The group - which wants strict sharia law - has carried out scores of attacks, mainly in the north-east.It has said it attacked several churches on Christmas Day, killing at least 42 people.Boko Haram has also in the past targeted international organisations and Muslim leaders from rival sects."There was a loud blast near the mosque just after the Friday prayers as people were trooping out of the mosque," one Maiduguri resident was quoted by the AFP news agency as saying."Everybody scampered to safety, leading to a stampede."Another witness said he had seen four bodies.Brig Gen Isa said there had been a "major incident" which had caused casualties but did not give any further details.Tens of thousands of people have fled their homes in Maiduguri and other cities which have been attacked by Boko Haram, especially in nearby Yobe state.Earlier this week, Nigeria's main Christian group warned that the community may have to defend itself if the security forces could not protect it.Analysts said the move raised the spectre of communal clashes in Africa's most populous nation, which is divided between a largely Muslim north and a mainly Christian and animist south.On Thursday, Nigeria's President Goodluck Jonathan summoned his security chiefs to discuss the Boko Haram threat.The leaders of Chad and Cameroon, which are close to Maiduguri, are reported to have held talks about how they can help prevent the violence spreading across their borders.The group, which originated in Maiduguri, this year staged suicide attacks on the headquarters of the UN and the national police in the capital, Abuja.It was also responsible for a string of bomb blasts in the central city of Jos on Christmas Eve 2010, as well as a New Year's Eve attack on a military barracks in the capital.