A female suicide bomber blew herself up in a college in northern Nigeria's biggest city of Kano, killing three people and critically wounding another seven in the fourth such attack by a woman in Kano in less than a week, a security source said.

The bomber targeted youths who were looking at a notice board for national youth service in Kano Polytechnic, the source told Reuters news agency on Wednesday.

Government spokesman Mike Omeri also confirmed the attack in a statement to the AFP news agency.

"At about 2.30 pm (1.30pm GMT), another female suicide bomber exploded an improvised explosive device at the Kano State polytechnic."

 The armed group Boko Haram, which is fighting for an Islamic state in religiously-mixed Nigeria, has repeatedly bombed Kano.

The armed group Boko Haram, which is fighting for an Islamic state in religiously-mixed Nigeria, has repeatedly bombed Kano as it targets areas away from its northeast heartlands.

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.

In a separate incident on Tuesday, two suicide bombers killed 13 people in attacks on two mosques in the town of Potiskum, in Yobe state in the northeast, a medical official there told Reuters.

Though much of the violence is concentrated in the remote northeast, they have struck across Nigeria in several bomb attacks since April.

On Sunday, they mounted a cross-border attack into Cameroon, killing at least three people there and kidnapping the wife of the vice prime minister.

Source: Agencies