Suicide bombings at bus stations in two major commercial centres in northern Nigeria have killed at least 27 people, police and hospital sources say. A bomber first struck in Potiskum, and hours later a second attack took place in Kano, the main northern city. Hospital sources said 17 people were killed in Potiskum while police put the number of dead in Kano at 10. Nigeria's President Goodluck Jonathan blamed militant Islamist group Boko Haram for the bombings. Its insurgency would "soon be over", as regional forces recapture territory from the group, he said in a statement. Boko Haram has not yet commented on the attacks. On Monday, Nigeria's military said air strikes were being carried out against the group in several areas, including Gwoza town where it first declared a caliphate last year, Reuters news agency reports.