Photos: More than 70 dead after fire rips through multi-storey building in South Africa

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

Another 52 were injured in what was on track to become one of the deadliest fires worldwide in recent years.

Bodies were discovered piled up at a security gate that was closed, preventing people from escaping the blaze, an official said.

City authorities said the building in a deprived crime-ridden area had been turned into illegal housing after being abandoned.

Most of those living there were foreigners, one resident said.

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

At least seven children were among those killed, the youngest under two years old, he said. Some were left burned beyond recognition.

"I'm grateful to be alive, there was a lot of us running, trying to find the fire exit and a lot of people eventually died because of the smoke inhalation," said Kenny Bupe, a survivor caught up in the blaze while visiting a friend.

The 28-year-old told AFP he was part of a group that managed to break open a locked fire escape gate and run to safety, while others "jumped out" of the windows to save themselves.

Blankets and sheets used for escape hung from the burned-out windows, a reminder of the dramatic scenes.

"This is a great tragedy, felt by families whose loved ones perished in this terrible manner," President Cyril Ramaphosa said at an event in the southern city of Gqeberha.

"Our hearts go out to every person who is affected by this disaster."

Mulaudzi said rescuers were combing the building "floor by floor", as firefighters damped down hotspots after putting out the flames.

Emergency services laid dead bodies under blankets on the street outside.

It was not immediately clear what caused the blaze.

Authorities said candles used for lighting inside the structure or stoves and other heating devices were a likely cause.

"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.

The building, which has been evacuated, is located in 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, which was cordoned off by police as a crowd of onlookers gathered.

Paramedics assisted survivors, some looking bruised and visibly in pain, as two women on a nearby street cried as they consoled each other.

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