At least 73 people were killed and dozens of others injured when a fire ripped through a five-story building in central Johannesburg Thursday.

Another 50 were injured, some suffering from smoke inhalation, and were taken for treatment at local hospitals, Emergency Management Services spokesperson Robert Mulaudzi said.

"The death toll is at 73 ... (including) seven children," Emergency Management Services spokesman Robert Mulaudzi said, with some left burned beyond recognition.

The youngest victim was aged less than 2 years old, he said.

Firefighters at the scene had put out the flames and search and recovery operations were ongoing.

"We are moving floor by floor conducting these body recoveries," Mulaudzi told local broadcaster ENCA.

An Agence France-Presse (AFP) reporter at the scene said emergency services were continuing to bring charred bodies out of the building, laying them down under blankets and sheets on the street outside.

"It is indeed a sad day for the city of Johannesburg ... over 20 years in the service, I've never come across something like this," Mulaudzi said.

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

Mgcini Tshwaku, a member of the city's mayoral committee in charge of public safety, said candles used for lighting inside the structure were a likely cause.

Security gate closed

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.

"A lot of people" might have been trapped inside when the fire started, he added, suggesting many in the building may have been squatting illegally.

"Inside the building itself there was a (security) gate which was closed so people couldn't get out," said Tshwaku.

"Many burned bodies were found stashed at that gate."

Fire trucks and ambulances were parked outside the red and white building with burned-out windows, which has been cordoned off by police, as a small crowd of onlookers gathered in the area.

Illegal occupation of disused buildings in the city center is widespread, with many said to be under the control of criminal syndicates who collect rent from occupants.

In June a blaze ripped through a dilapidated building in Johannesburg and killed two children under 10 who were locked in an apartment.