JOHANNESBURG, Dec. 29 (Xinhua) -- Four members of a South African family were killed when their six-seat airplane crashed in Harrismith in the country's Free State province on Monday afternoon, the South African Press Association (SAPA) reported.

The plane, a Beechcraft Bonanza, crashed into the side of the Platberg Mountain in thick mist, Netcare 911 spokesman Chris Botha was quoted as saying.

"Sadly, all four passengers on board the plane died," he said.

According to the Beeld newspaper, the victims were Johan Adriaan Venter, an orthodontist from Vereeniging, his wife Elzab, both aged 61; their son Christiaan and his wife Christine, both aged 29 and both chartered accountants.

Santjie White of the South Africa Search and Rescue Organization said the aircraft, en route from Vereeniging to Margate, diverted to Harrismith due to a technical problem.

"Our aeronautical rescue coordination center in Johannesburg was activated after the problem was reported. Contact was lost with the aircraft and shortly thereafter the wreck was spotted by an over flying aircraft when the weather cleared sufficiently."

The airplane crashed into a nearly inaccessible part of the mountain.

Paramedics, who were notified about the crash at 1:30 p.m., had to walk to the wreckage.

The South African Civil Aviation Authority's Phindile Gwebu said a helicopter with rescuers was dispatched to the scene for a quicker response.