JOHANNESBURG (AFP) - A South African prisoner has died after warders assaulted him in an incident journalists witnessed at a maximum security jail, a senior prisons official said Friday.Journalists saw guards beat the inmate while on a guided tour Wednesday with lawmakers at the Groenpunt jail in the central Free State province, where dozens of prisoners rioted last week.He died hours after the attack, the province's deputy prisons commissioner Grace Molatedi told AFP.A group of prisoners had launched an attack on a warder, and other officials stepped in to break up the fight, she said."Yes, an offender has died. This offender that has died was one of the three offenders that attacked ... and stabbed an official," Molatedi told AFP.He died at a prison hospital a few hours after the incident.She said the exact cause of death was unclear and officials were awaiting post-mortem results.Molatedi had earlier told local daily The Star that officials had used "necessary force" to restrain the prisoners from attacking a fellow warder."They intervened and used necessary force to prevent the officers from further stabbing or possibly killing their own," Molatedi told the paper. "It was in the process of them using necessary force that the dead offender was injured," she said.Prison officials detained the journalists, who managed to photograph the attack of the one inmate through a fence. They confiscated the journalists' cameras and mobile phones and deleted all images, local media reported earlier.Last week's riot at the prison left dozens wounded as the rampaging inmates set their cells on fire in protest over conditions at the facility.