At least 64 people have died in a fire that engulfed a five-storey building in central Johannesburg on Thursday, the South African city's emergency services said.

"We are now on 63 fatalities and 43 people injured who were transported to various healthcare facilities," said Emergency Management Services spokesman Robert Mulaudzi.

Dozens of others were injured, some suffering from smoke inhalation, and were taken for treatment at local hospitals, Emergency Management Services spokesman Robert Mulaudzi said.

"We're on 64 bodies, which we have recovered and also 43 people who sustained minor injuries," Mulaudzi told local broadcaster ENCA.

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Firefighters who were called to the scene put out the flames and search and recovery operations were ongoing.

"We are moving floor by floor conducting these body recoveries," Mulaudzi said, adding the death toll was expected to rise.

It was not immediately clear what caused the blaze, which broke out overnight.

The building, which has been evacuated, is located in a deprived area of what used to be the business district of South Africa's economic hub, and was used as an informal settlement, Mulaudzi said.

Television footage showed fire trucks and ambulances outside the red and white building with burned-out windows, which had been cordoned off by police.