Three security guards were burnt to death and one was shot dead in a cash heist in Limpopo's Waterberg area, south of Polokwane, on Friday morning, police said.

The guards were transporting money to a bank in Lephalale when four gunmen in a Mercedes-Benz pushed their van off the road, causing it to overturn, said Superintendent Ronel Otto.

The robbers fired several shots at the van, killing one of the guards, before stealing an undisclosed amount of cash and then dousing the van with a flammable chemical.

Before the robbers fled, they set the van alight, with all four security guards trapped inside.

The remaining three guards were burnt to death in the blaze.

The robbers left the car they used to force the security vehicle off the road behind, and fled in two other vehicles (a BMW and a Mercedes-Benz).

The car that had been left behind was also set alight.

"The BMW which the suspects used to flee the scene was found abandoned in the Bakenberg area (near central Limpopo)," said Otto.

She said no arrests were made and cases of armed robbery and murder would be opened for investigation.

Limpopo police commissioner Calvin Sengani condemned the incident, saying "No stone will be left unturned to trace the suspects and bring them to book."

Sengani appealed to the public to assist the police.

"Anyone with information can phone the Crime Stop number at 08600-10111."

Meanwhile the office of the national commissioner of police authorised a team of investigators to fly to Limpopo to assist in the investigation.

"Police management has reacted to the news of the barbaric murder of four security guards with anger and revulsion... (We) will assist their counterparts in the province in hunting down these inhumane killers and bringing them before the courts," said national police commissioner Jackie Selebi.

A reward of R500 000 was also offered for information leading to the arrest and conviction of the robbers.

Selebi conveyed his condolences to the relatives of the murdered guards. - Sapa