Suspected Islamists drove a car packed with explosives into a police patrol in the northeast city of Maiduguri, killing five policemen, a police source and a witness have said.
A police sergeant, who declined to be named because he was not authorised to speak, said the bombers crashed a Volkswagen Golf into the parked patrol vehicle and detonated the explosives by the Dalori Quarters, an estate for civil servants on the outskirts of the city.
Aliyu Surajo, 66, who witnessed the events from in front of his house, said there was a loud blast, then smoke filled the air. He said he saw five charred bodies of policeman in their vehicle afterward, and two bodies in the bombers' car
The police spokesman for Borno state, of which Maiduguri is the capital and the birthplace of the insurgency, did not respond to a request for comment.
There was no immediate claim of responsibility. Boko Haram, an al Qaeda-linked group fighting to reinstate a medieval Islamic caliphate in northern Nigeria, has sometimes used suicide bombings as a tactic.
Security sources say Boko Haram has killed hundreds, possibly thousands, this year in a campaign of violence that is growing in intensity - and has spread beyond its traditional targets in the government and security forces to civilians.
Suspected Islamist militants detonated a bomb in a crowded marketplace in northeastern Nigeria late on Saturday, killing at least 20 people.
A military crackdown since last May has failed to quell the insurgency, which after four-and-a-half years remains the leading security threat to Africa's top oil producer, although it remains far from southern oil fields or the commercial hub of Lagos.