JOHANNESBURG (Reuters) - Six South African miners were burnt to death when the bus taking them to work was set alight by a petrol bomb thrown by unknown attackers, police and the National Union of Mineworkers (NUM) said on Tuesday.

The platinum belt in the world’s top producer of the precious metal has been a flashpoint of violence rooted in community grievances over jobs and revenue flows and conflict between rival unions.

“Six workers were burned beyond recognition and the other workers had to escape through windows. We also do not know what is the motive of the attack,” Phillip Mankge, the NUM’s North East Regional Secretary, said in a statement.

The attack occured Monday night near the town of Burgersfort in northern Limpopo province while the workers were being driven to the Modikwa platinum mine, operated by African Rainbow Minerals (ARM) and Anglo American Platinum.

ARM spokeswoman Jongisa Magagula said production at the mine, which produces around 300,000 ounces of platinum group metals annually, had halted and staff would hold a mass meeting later in the day.

The companies said in a joint statement that 44 workers were also receiving medical attention in the wake of the attack.

The South African Police Service said that it was offering a 500,000 rand ($42,000) reward for information leading to arrests or possible convictions of those responsible.

When police arrived on the scene “the bus had already burned to ashes with six victims inside,” police said in a statement. A high level team of investigators had been assigned to the case.

NUM said that there have been “a number of incidences in Burgersfort and surrounding areas recently that led to road closures and trucks burned.”

A turf war between NUM and arch rival the Association of Mineworkers and Construction Union (AMCU) has also triggered violence that has killed dozens of workers, including 34 wildcat miners shot dead by police in 2012 outside the Marikana mine operated by platinum producer Lonmin.