The death toll in a building collapse in the Nigerian city of Lagos city has gone up to 12.

The dilapidated building, which housed a primary and a nursery school, came down without warning on Wednesday.

The state emergency management agency said scores of people are still trapped underneath the rubble, with no fewer than 20 school children among them. The cause of the collapse is yet to be determined.

Lagos state governor, Akinwunmi Ambode said the building, which had been marked for demolition, is classified as residential and the school was operating illegally on the top two floors.

The governor promised a thorough investigation and vowed that those responsible will be prosecuted.