Addressing the media on Thursday, Panyaza Lesufi, the premier of Gauteng province, which includes Johannesburg, said 12 casualties remain in hospital.

Emergency services are still on the scene assessing the damage and evacuating people where necessary.

Several streets have been closed to traffic in what is usually one of the busiest parts of South Africa's commercial hub.

Gauteng officials have said it is still unclear what led to the massive explosion but suspect it may have been cause by a rupture of some underground gas pipes.

On Wednesday evening, witnesses reported feeling the ground shake then hearing a loud bang moments before the earth opened, and a section of Breet street collapsed.

The blast overturned cars and minibus taxis. Video footage shows a stretch of the road which was split wide open from the incident.