JOHANNESBURG: At least 31 people were believed to have died in a gas explosion in a disused mine shaft in South Africa that happened last month but was only now coming to light, authorities said on Friday.

The suspected illegal miners were believed to have been killed on May 18 and most of their bodies were still underground, South Africa's Department of Mineral Resources and Energy said.

Authorities were being prevented from sending a search team into the mine to retrieve bodies because there were still dangerously high levels of methane gas in the shaft, the department said in a statement.

It said it was considering various options to "speedily deal" with what was "a unique and strange situation."

The government department said it had received information that three bodies had been recovered after they were brought to the surface by other illegal miners. Another 16 suspected illegal miners who were also in the shaft have handed themselves over to authorities, police said.

Authorities said they believe the miners are nationals from neighboring country Lesotho. Lesotho's foreign ministry recently passed information onto South African authorities on the incident.

The mine, which was previously owned by Harmony, South Africa's largest gold-mining company, was last operational in the 1990s, the mineral resources department said. It is located in the city of Welkom in the central Free State province.