An Egoli Gas pipeline exploded in City Deep along Vickers Road, south-east of Johannesburg, Gauteng paramedics said on Wednesday.
"It exploded at around 10.30am last night (Tuesday)," Johannesburg Emergency Services spokesperson Percy Morokane said.
"It appears that one of the connectors on the pipeline leaked which then led to the explosion."
He said about 30 firefighters from five stations around Johannesburg were on the scene battling the blaze caused by the explosion.
"It is a pretty hazardous operation, complex and delicate. There are concerns after the initial impact assessment that if technicians close the pipeline off, it may lead to another explosion."
Firefighters estimated that it would take between four and five hours to bring the inferno under control. Vickers Road was completely closed off.
No injuries were reported and emergency personnel were standing by.
Egoli Gas was not immediately available for comment. - Sapa