Two prisoners escaped but were recaptured, the spokesman said.

The police were at the scene of the blast, and the wounded have been rushed to hospital under heavy guard.

Police are investigating a case of murder and illegal possession of weapons, confirming dangerous criminals were part of the escape plot, says the BBC's Nomsa Maseko in Johannesburg.

The explosion took place just metres from the prison, as the bus carrying the prisoners returned from court in Randburg, about 40km (25 miles) away, adds our reporter.

The driver heard a loud bang just outside the prison, South Africa's Eyewitness News quoted a police spokesman as saying.

"There was an explosion in the vehicle. Two of the prisoners managed to escape, but they were rearrested," said the spokesman, Neville Malila.

Early reports suggest that plans to detonate an explosive device at the back of the van to facilitate an escape went badly wrong.

South Africa's National Commissioner of Correctional Services Tom Moyane described the incident as South Africa's most "daring escape attempt".