At least 19 fruit pickers have been killed after a freight train ploughed into their truck as they travelled to work in South Africa.

The victims' vehicle was flung over 200 metres in the smash, which took place on a level crossing leaving blood and body parts strewn across a wide area of open land.

"It is a very gruesome scene. Some bodies are without heads and some without limbs. Forensic teams are still working on the scene," police spokesman Joseph Mabusa said.

The carnage was so gruesome that forensic teams struggled to work out exactly how many victims there were.

Emergency services said at least 24 other people were injured, some of them critically, in the crash near the town of Hectorspruit, about 400km east of Johannesburg.

"The driver was taken to hospital. His condition is unknown. It's difficult to say what happened but at this stage it seems that the truck miscalculated as it was crossing the railway line," added Mr Mabusa.