Suspected Islamic militants attacked a police patrol in northeastern Nigeria, wounding at least three officers.
Major General Jack Nwachukwu Nwaogbo said the attackers threw a bomb at the patrol in the city of Maiduguri Wednesday.
Witnesses report the blast was followed by sporadic bursts of gunfire.
Officials are blaming the Boko Haram sect for Wednesday's incident, as well as a series of other that have killed at least 40 people in the past two weeks.
Boko Haram's name means "Western education is sinful" in the Hausa language. It wants to spread strict Islamic law across northern Nigeria.
Nigerian officials recently announced the arrest of 100 suspected Boko Haram members for carrying out a series of bombings across the country's north. They said the suspects would undergo "deradicalization" and "perception management" programs so they can return back to society.
Boko Haram has refused President Goodluck Jonathan's offer of formal talks to end the violence. The group does not recognize the Nigerian constitution and says it wants an independent country in the north ruled by Islamic law.
Some information for this report was provided by AP, AFP and Reuters.