Suspected Islamist militants opened fire on a school in Nigeria's northeastern city of Maiduguri on Tuesday, June 18 killing nine students, witnesses and a medical worker said, the second deadly attack on schools in three days, Reuters reported.

Witness Ibrahim Mohammed said he was taking exams in a classroom at Ansarudeen School when gunmen stormed the building, opening fire at random.

Mortuary attendant Alhaji Baba at the State Specialists morgue in Maiduguri told Reuters he counted nine corpses come in after the attack.

Boko Haram, whose name roughly translates as "Western education is sinful" and is seen as the biggest security threat to Africa's top oil producer, has attacked several schools in the past.

Seven students, two teachers and two insurgents were killed when suspected Boko Haram militants attacked a school in the northeastern town of Damaturu on Sunday.

The two attacks have raised fears that a month-long offensive by government troops has merely pushed militants into hiding, from where they can still launch devastating operations.

Insecurity in the north caused by the Boko Haram insurgency has been a boon for criminal gangs and rival ethnic militias with scores to settle, security sources say.