A police station was bombed late Tuesday in the northern Nigerian city of Kano, four days after Islamic insurgents killed 185 people in a series of terror attacks in the city.

There is no word on casualties from the bombing and no one has yet claimed responsibility.

Earlier, Kano police discovered 10 car bombs and hundreds of other explosives in the city.

Human rights groups say the militant Islamic group Boko Haram has killed 935 people since 2009, using violence and terrorism to try to turn northern Nigeria into a conservative Islamic state.

It has said it would kill Christians in the mostly Muslim north.

Nigerian President Goodluck Jonathan has declared a state of emergency in much of the north. U.S.-based Human Rights Watch is urging him to take further steps against the militants, including beefing up the police presence.

Human Rights Watch says Boko Haram's sophisticated and deadly attacks show a complete disregard for human life.