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

Six other people were critically wounded by the bomber, who targeted youths looking at a notice board for national youth service at Kano Polytechnic, government spokesman Mike Omeri said.

There was no immediate claim of responsibility, although militant group Boko Haram has repeatedly bombed Kano as it radiates attacks outwards from its northeast heartlands.