Armed men have shot dead a South African policeman in an ambush on a convoy carrying prisoners, allowing twenty-one of them to escape.

Another policeman was seriously wounded.

The prisoners were being taken to a court in the Soweto township, which adjoins Johannesburg.

The BBC correspondent in the city says it's the latest in a series of escapes from custody which are embarrassing the South African government.

On Christmas Day eleven prisoners serving long sentences for serious crimes escaped from a Johannesburg jail, one of them for the sixth time.

The prison authorities say they've radically reduced the number of escapes in the past two years.