The death toll from a gas tanker explosion in Johannesburg on Christmas Eve has climbed to 18, officials said on Tuesday, as South African President Cyril Ramaphosa promised to support people affected by the blast.

The explosion tore the roof off the emergency department at the Tambo Memorial hospital, destroyed two houses, several cars and injured bystanders up to 500 metres from the scene in the city's Boksburg suburb.

The vehicle caught fire under a low bridge. The truck driver has been arrested on suspicion of culpable homicide, police said.

"Of the 18 deceased people, nine are Tambo Memorial Hospital staff," the Gauteng Department of Health said in a statement.

President Ramaphosa said hospital patients and children were among the dead.

"The nation's hearts go out to everyone affected by this devastating incident," he added in a statement.

"While we await the outcome of investigations into this tragedy, this incident does... call on all of us to show due care and to avoid risk when we are out on our roads." (Reporting by Bhargav Acharya; Editing by Andrew Heavens)