A fire that engulfed a five-storey building killed more than 70 people including children in central Johannesburg on Thursday, the South African city's emergency services said.

Another 52 were injured, some suffering from smoke inhalation, and were taken for treatment at local hospitals, Emergency Management Services spokesman Robert Mulaudzi said.

"We have now 73 fatalities and 52 people injured who were transported to various healthcare facilities for further medical care," Mulaudzi said.

At least seven children were among those killed by the flames, in what is on track to become one of the deadliest fires worldwide in recent years.

The youngest victim was less than two years old, he said. Some were left burned beyond recognition.

Firefighters at the scene have put out the flames and are "busy with damping down", while search and recovery operations are ongoing, Mulaudzi said.

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

An 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, suggesting many may have been squatting there 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 centre is widespread, with many said to be under the control of criminal syndicates who collect rent from occupants.

Authorities estimated more than "80 shacks" were set up inside.

"The fire spread very quickly affecting different levels of the building because of the combustible materials used," said Mulaudzi.

The blaze was the deadliest in South Africa in recent years and one of the worst worldwide.

In December last year, a fuel tanker blast near Johannesburg killed 34 people, while in June, flames ripped through a dilapidated building in the city and killed two children under 10 who were locked in an apartment. (AFP, excluding headline)

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