Gunmen have attacked a military-run secondary school near Kano in northern Nigeria, killing four people, hospital sources say.They said the dead were air force personnel, with two more injured.A local farmer told AFP that he could hear gunshots from the school which lasted 30 minutes. Residents in other areas of Kano also reported gunfire.Northern Nigeria has been suffering violence from Islamist militants, but Kano has not been targeted recently.A doctor at the local hospital said that six air force personnel arrived on Thursday night with gunshot wounds following the incident at the school. Three were dead on admission and another died later, he said.The Islamist group Boko Haram has carried out a wave of bombings and killings in Nigeria in recent years.In August it claimed responsibility for a bomb at the UN headquarters in Nigeria's capital, Abuja, which killed at least 23 people.There has been no claim of responsibility for Thursday's incident.