They are believed to be from neighbouring Lesotho – which reported the incident to South African authorities – and died in a ventilation shaft in Virginia mine in Welkom city, the department of mineral resources and energy (DMRE) said.

It was not clear what caused their deaths but the DMRE said an investigation into the incident with the help of Harmony Gold – the previous owner of the mine which ceased operations in the 1990s – determined that methane levels were very high in the ventilation shaft.

“As such, it is currently too risky to dispatch a search team to the shaft. However, we are considering various options to speedily deal with the situation,” it said.

Harmony said it viewed the incident – which occurred on May 18 – as a criminal matter and has handed it over to the South African police for further investigation.

“Harmony is working with the DMRE to provide direction on the next steps,” said spokesman Sihle Maake.

Methane is a highly explosive greenhouse gas often found in underground mines and poses serious safety and health threats to mine workers.