President Xi Jinping extended on Friday a message of sympathy to South African President Cyril Ramaphosa over a deadly fire that ripped through an apartment building in central Johannesburg and killed at least 74 people, including 12 children.

Another 61 people were injured and received hospital treatment in what was one of the deadliest fires worldwide in recent years.

It was not immediately clear what caused the blaze. Authorities said candles used for lighting inside the building or stoves and other heating devices were a likely cause.

In the message, Xi, on behalf of the Chinese government and people, conveyed sympathy to the families of the victims and those who were injured, saying that China firmly stands with the government and people of South Africa in the face of this disaster.

He expressed belief that under the strong leadership of President Ramaphosa and the South African government, the people of South Africa will surely overcome this difficulty.

The building, which has since 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 by people squatting there illegally, authorities said.

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

Ramaphosa visited the scene on Thursday evening and called for an investigation into what caused the blaze so that such incidents won't occur again.

He said authorities were working "full out" to assist those affected, adding that the disaster needed to be investigated.

"It's a wake-up call for us to begin to address the situation of housing in the inner cities," Ramaphosa said.

Ndumiso Mlilo is a freelance journalist for China Daily.