Children aged between 13 and 15 began fleeing their three-story school building in the city of Kaduna on Monday after it filled with smoke coming from a carpentry workshop next door, said Kaduna police spokesman Saad Yahaya.

The children mistakenly believed their building had caught fire. The carpenter was burning trash.

"The kids started running helter-skelter for their dear lives," Yahaya said. The children who died "were trampled along the staircases," he added.

Yahaya said 12 boys and girls from the All Saints International College have been confirmed dead, while another nine were hospitalized.

A Nigerian classroom