Almost 1,000 workers remained trapped underground at a South African gold mine after a power failure.

Sibanye Gold Ltd. is using a generator to get the workers out of the mine, but “having some problems,” according to spokesman James Wellsted. Rescue workers have freed 336 employees from two shafts, while 955 remain trapped at another shaft, he said, adding that there were no fatalities and the workers are fine.

“We don’t know when it will be done,” Wellsted said. “They are trying their best to get the power restored.”

South Africa’s state-owned utility said power will soon be returned to the Beatrix mine. The outage was caused by a severe thunderstorm that swept through large parts of northern Free State province, said a spokesman for Eskom Holdings SOC Ltd.

Mine safety is a perennial concern in South Africa, which operates some of the world’s deepest and most dangerous mines. Workers are having to go deeper in ageing shafts to access additional ore in a country that’s been mined commercially for over a century. Last year, fatalities in the sector increased last year for the first time in a decade.

The National Union of Mineworkers branch leaders at Beatrix are assisting in taking food and water underground, the union said Thursday. The incident began at about 8 p.m. on Wednesday during the night shift, NUM spokesman Livhuwani Mammburu said by phone earlier.

Officials from the Department of Minerals are on site and providing advice, the department said on Twitter. “Currently all employees still underground are accounted for,” it said.

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