"The fire operation continues at the two-storey building that was engulfed with flames earlier this morning," the City of Johannesburg Emergency Management Services said in a statement.
A spokesperson for the city's emergency management services told television channel eNCA it was unclear how many people were still trapped inside the building.
An investigation into the cause of the fire will be undertaken once firefighting operations have been completed, officials said.
More than 70 people died last August after a fire raged through a run-down, five-storey Johannesburg apartment block, in one of the worst such disasters in a city where poverty, household fires and homelessness are widespread.
An inquiry to determine responsibility for that blaze is ongoing, but the incident highlighted a longstanding housing crisis that has seen some buildings in the inner city of Johannesburg occupied illegally.
(Reporting by Olivia Kumwenda-Mtambo; Editing by Frances Kerry)
