The Department of Mineral Resources and Energy (DMRE) reported Friday that at least 31 suspected illegal miners died last month in a ventilation shaft at the closed Virginia mine in Free State province.

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Methane levels were very high in the ventilation shaft and an explosion had occurred at the mine last operational in the 1990s, according to an investigation by the DMRE in conjunction with Harmony Gold (HARJ.J), South Africa's largest gold mining company and previous owner of the mine.

The DMRE said in a statement that sending a search team to the shaft would be too risky, as current methane levels in the mine's ventilation shaft remain high. "However, we are considering several options to quickly address the situation," the department said. 

"All stakeholders will endeavor to ensure that the deceased suspected illegal miners are brought to the surface," it added. 

According to officials, South Africa and Lesotho are in talks to recover the bodies and bring them home. A spokesman for Mining and Energy Minister Gwede Mantashe said it could take a while to recover the bodies, as authorities did not want to risk losing any more lives.

According to the DMRE, the suspected illegal miners are believed to be Lesotho nationals and died in Free State province on May 18. The Lesotho government informed South Africa of the deaths on Thursday.

Harmony's head of investor relations, Jared Coetzer, said that the shaft, acquired in the mid-1980s, was shut down shortly thereafter due to a methane gas explosion and was never used by the miner.

In South Africa, one of the world's largest producers of gold and other precious minerals, informal mining is rampant in old mine shafts that lie abandoned decades after the country's gold rush.