At least 31 people are believed to have died in a methane explosion at an abandoned gold mine in South Africa last month, officials said on Friday.

The incident occurred on May 18 in a ventilation shaft at the Virginia gold mine outside Welkom, some 260 kilometers (160 miles) from Johannesburg.

But the details of what the government called "a unique and strange situation" are only now coming to light.

Government planning search operation

South Africa's Department of Mineral Resources and Energy (DMRE) determined that methane levels were abnormally high in the ventilation shaft of the mine and an explosion had occurred.

"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," the DMRE said.

The victims are believed to be "zama zamas," or illegal gold miners, from neighboring Lesotho.

Thousands of "zama zamas" work in South Africa, scouring abandoned mines for gold and other minerals. They often work in dangerous conditions.

"We are receiving reports that other suspected illegal miners have assisted in retrieving in at least three bodies, so 28 are believed to still be underground," a DMRE spokesperson told the AFP news agency.

According to the mineral resources department, the mine was previously owned by Harmony Gold — chaired by Patrice Motsepe, one of South Africa's richest men and the brother-in-law of South African President Cyril Ramaphosa — but has been abandoned since the 1990s.

Lesotho calls for bodies to be repatriated

Lesotho's Foreign Ministry said it had alerted South Africa's High Commission in Maseru as early June 10, but it was unclear why South Africa only announced the incident weeks later.

A spokesperson for Lesotho's prime minister said the country is now in talks with South Africa to retrieve the bodies and bring them home.

Makhabane Peete, a traditional chief from the Berea district where most of the victims hailed from, spoke on behalf of the bereaved families and said he had lost six of his men.

"We don't want anything more than just to have those bodies retrieved and repatriated, that's all we are asking for," he said.

zc/rs (Reuters, AFP, AP)