A suicide bomber blew herself up at a college in northern Nigeria's biggest city of Kano today, killing six people in the fourth such attack there in less than a week.

Six other people were critically wounded by the bomber who targeted youths in Kano Polytechnic, government spokesman Mike Omeri said.

There was no immediate claim of responsibility, although militant group Boko Haram, which is fighting for an Islamic state in religiously-mixed Nigeria, has repeatedly bombed Kano.

Using female suicide bombers in the city appears to be a new tactic of Boko Haram, although they have used them on occasion for years in the northeast.

Two female suicide bombers blew themselves up at a trade show and a petrol station in Kano on Monday, killing one other person and injuring at least six others.

On Sunday, a female suicide bomber killed herself but no one else while trying to target police officers.

Meanwhile, Nigerian forces have arrested two Boko Haram suspects who were travelling with a ten-year-old girl with explosives strapped to her.

The suspects had been intercepted in a Honda CRV car travelling along a road in the north's Katsina state.

"Ten-year old Hadiza was discovered to have been strapped with an explosive belt and immediately, Iliya and Zainab made attempt to escape with the car, but were later blocked by other concerned Nigerians and subsequently arrested," Mr Omeri said.