Fourth female suicide bomber hits Nigeria's Kano, kills six

A female suicide bomber blew herself up in a college in northern Nigeria's biggest city of Kano on Wednesday, killing six people and critically wounding another six 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 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 as it radiates attacks outwards from its northeast heartlands.

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.

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 on Wednesday.

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.