Rescue efforts are under way in Nigeria after a three-storey school building collapsed while classes were taking place.

Scores of children are thought to be inside the building.

Associated Press video from the scene showed at least one dust-covered child being carried out of the rubble.

People help a child after he was rescued from the scene of a building collapse in Lagos
People help a child after he was rescued from the scene of a building collapse in Lagos (AP)

People were crowded around the scene in the densely populated neighbourhood in Lagos, Nigeria’s commercial capital.

They cheered as the child was lifted out.

An emergency management official said more than 40 people had been found among the ruins. It was not clear how many had died.