Witnesses in the northern Nigerian city of Kano say they have heard a blast at a police station, followed by gunfire.

Police in the region said they had received reports of the violence but did not confirm the attacks.

The were no immediate reports of casualties or claims of responsibility.

Kano, the second-largest city in Africa's most populous country, has been plagued in recent months by violence which authorities blame on the radical Islamic sect known as Boko Haram.

The group has claimed responsibility for a series of coordinated explosions that killed 185 people in Kano last month, and a set of Christmas Day attacks on several churches.

Boko Haram has said it wants strict Islamic law to be imposed across Nigeria, which is divided between the mostly Muslim north and largely Christian south.

President Goodluck Jonathan has declared a state of emergency in parts of the north, the site of most of the violence.

Nigeria has endured spates of sectarian violence over the years, although some of the conflict stems from economic and political factors as well.