Seventy-three bodies were recovered by emergency crews from a burnt five-story building in Johannesburg Thursday after one of the worst fires to hit the South African city.

The fire also sent 52 people to local hospitals for smoke inhalation, Emergency Management Services spokesperson Robert 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, Mulaudzi said. Some were left burned beyond recognition.

Search and recovery personnel were moving floor by floor after firefighters put out the flame, Mulaudzi told local broadcaster ENCA.

An Agence France-Presse 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.

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.

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.

 WITH AFP