ER24 spokesperson Andre Visser said details were slim.
"The accident happened some time between 7am and 8am this morning," he said.
A goods train struck a truck, which had been carrying farm workers in a container similar to the ones used in shipping, at a level crossing near Hectorspruit in Mpumalanga, he said.
The truck collided with the train at the Tulluh level crossing at around 7am, Netcare 911 spokesperson Jeff Wicks said.
"The force of impact saw the truck cleaved and flung from the railway line," Wicks told the Mail & Guardian.
He said the cause of the accident was unclear and would be investigated by the police.
Thulani Sibuyi, head of eastern Mpumalanga province's community safety department, earlier told Agence France Press that at least 30 people had been killed.
Victims had been "torn into pieces" and parademics were still on the scene east of the provincial capital Nelspruit, he said.
"It would appear as if the truck driver may have crossed the railway line without having a proper look-out, and as a result the train hit him and then pulled [the truck] for about a kilometre to two kilometres," said Sibuyi.
Paramedics, ambulances and a helicopter are at the scene and the the injured are being taken to various facilities in the region.
The driver of the train was uninjured.
Sandile Simelane, a spokesperson for Transnet Freight Rail, said the train did not derail and there was no damage to the line itself. Services on the line will return to normal once the police have taken statements and completed a forensic investigation of the scene.
Simelane said that although there are no booms at the level crossing where the accident took place, there is adequate road signage.
"We have adequate signage at all our level crossings and normally as a train approaches a level crossing, within a kilometre or two, it starts blowing a horn to warn people [of its approach]," he said. The train continues to blast its horn until it has moved well past the level crossing.
Simelane appealed to the public to be vigilant and to take care when approaching level crossings.
"Trains are very dangerous and if you do not comply with road traffic signs, it is criminal and could lead to what we've just experienced," he said. "Trains can kill, they do kill as we've just seen."
He expressed sincere condolences to the families of those who lost their lives in the accident. – Additional reporting by Sapa