Two firefighters remain trapped in a government building in the Johannesburg CBD, where a fire broke out on the upper floors earlier on Wednesday.

Four firefighters have made it to the ledge where they are waiting to be rescued by a helicopter.

"Our rescue unit is still looking for 2 more firefighters who are believed to be trapped inside the building," MMC for Public Safety, Michael Sun told the media outside the building.

An alleged electrical fault on the 23rd floor is believed to have caused the fire according to the staff who were evacuated out of the building when the fire started. They claim this is the third fire to have occurred at the building this year. They say the building is a safety hazard, and they had previously complained about the electrical fault.

The building is shared by the departments of health, human settlements and co-operative governance.

Earlier a firefighter fell to his death while battling the blaze.

"When the fire started, our firefighters went in and unfortunately one firefighter slipped from the building and fell to his death," Sun told News24.

A staffer, who wished to remain anonymous, told News24 that the fire started at the health department's side of the building.

The woman said about three or four firefighters went into the building as it was being evacuated.

While outside, she witnessed windows exploding and a firefighter rolling on the ground.

News24