At least 74 people have been killed when a fire raged through a five-floor Johannesburg apartment building.

The building was gutted, blackened by soot and still smouldering on Thursday as emergency services gathered around it and bodies lay covered in blankets on a nearby street.

At least 12 of those killed were children, the youngest a one-year-old, according to city and medical officials.

They said at a news conference that an undetermined number of people were still missing and many bodies recovered were burned beyond recognition.

More than 50 people were injured, six of whom were in a serious condition in the hospital. Emergency services officials had earlier warned that the death toll could rise as they continued to search the scene more than 12 hours later.

Authorities have not established the cause of the fire but Mgcini Tshwaku, a local government official, said initial evidence suggested it started with a candle.

Inhabitants used candles and fires for light and to keep warm in the winter cold, he said.