Nigeria's Boko Haram blamed for Maiduguri attack
A blast has rocked an area near a mosque in the restive north-eastern Nigerian city of Maiduguri.
Witnesses said four people were killed but a military spokesman said there were two deaths, in a shootout between robbers and security forces.
Another army spokesman told the BBC the Islamist group Boko Haram was behind a "major incident" in the city.
Maiduguri has been plagued by attacks from Boko Haram, which wants to impose strict Sharia law across Nigeria.
The group has said it attacked several churches on Christmas Day, killing at least 42 people.
"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.
A military spokesman, Lt Col Hassan Mohammed said armed robbers had attacked a market near a mosque, killing two people and critically injuring a third.
Another military spokesman, Brig Gen Isa, told the BBC there had been a "major incident". He gave no details of casualties, but said the army was holding Boko Haram responsible.
The group has been accused in the past of carrying out robberies to raise funds.
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.