At least 15 people have been killed, and about 40 injured, by a huge fuel tanker explosion near a hospital east of Johannesburg.

The tanker appeared to have got stuck under a low bridge on Saturday morning in Boksburg city, about 100m from Tambo Memorial Hospital.

Patients were evacuated from the hospital's casualty after part of the roof collapsed following the blast.

The truck was carrying liquid petroleum gas.

"Apparently, a gas tanker drove under the subway bridge and got stuck in there, and due to friction, it caught alight," emergency services spokesperson William Ntladi was quoted as saying.

As firefighters tried to put out the blaze, there was a huge second explosion - captured on amateur video - that destroyed a fire engine and two motor vehicles.

There are fears the death toll could rise, as 19 people are in critical condition and 15 others are stable but seriously hurt, AFP reports.

Eyewitness Michael Kulinji described the blast as "more like a bomb" and told the Reuters news agency that he had seen the fire under the bridge.

Another witness, Jean Marie Booysen, said that she saw "immense flames at about 06:35 local time (04:35 GMT) and thought "this feels like 6.5 on the Richter", according to Reuters.

Footage apparently taken in the immediate aftermath of the blast shows a number of dazed and burned people stumbling around a residential area near the epicentre.