"There's a cordon around the entire area and we are being told by military officers on the ground that their operations are ongoing," he said.

"A group of people stormed the police station with automatic weapons ... subsequently the militants left the police station and set up in an open piece of ground next to it.

They have now been surrounded, it seems, by the military forces here." The attack was the second on a police station in northern Nigeria's biggest city in a week.

A divisional police officer was killed in the Sharada district last week. Kano is one of 12 northern Nigerian states which introduced sharia law in 2000.

The move by state governors alienated Christian minorities and sparked violence.

Mike Hanna said it was not clear whether the attack was related to state and national elections being held in Nigeria.

"There is a precedent for this type of incident, we are told. We are not sure whether or not is related to the ongoing elections in the country ... the timing of this would appear to indicate some kind of link to the electoral process," he said.