The government said it was launching an investigation into the accident at Harmony Gold’s Doornkop mine, the worst such incident in South Africa since 2009.

Rescue teams on Wednesday brought eight miners to the surface unharmed after the blaze trapped 17 workers at Doornkop, leaving nine still missing.

But on Thursday the company announced that rescue workers had located the bodies of eight more men.

“The search continues for the ninth employee,” Harmony Gold said in a statement.

The bodies were discovered during rescue work late Wednesday at the mine west of Johannesburg, National Union of Mineworkers (NUM) spokesman Livhuwani Mammburu told AFP.

An earth tremor is blamed for the accident some 1.7 kilometres (more than a mile) below ground when a rockfall damaged electric cables and started the fire, the mine owners and the NUM said.

“The fire is still burning but under control,” NUM health and safety official Erick Gcilitshana said.

Eight workers were saved on Wednesday after taking shelter in a refuge chamber, but Harmony Gold said that access for rescue workers had been hampered by smoke and the shift in terrain.

Company chairman Patrice Motsepe described the accident as “a sad day for the industry”.

“We were confident that they were going to come out alive,” he told journalists.

Mining Minister Susan Shabangu promised a probe into the “deeply regrettable” incident.

“This tragic accident takes us back… the industry has in the past years worked hard to ensure safety,” she said in a statement.

“We must ensure that we do all we can to get to the bottom of what caused this incident, in order to prevent similar occurrences in future.”

Accidents remain frequent in South Africa’s ultra-deep mines despite efforts to reduce fatalities.

In July 2009, nine workers were killed in a rock fall in a platinum mine. The same year, 82 illegal diggers died in an disused gold mine shaft when a fire broke out underground.

In all, 128 miners were killed in 2010, according to the latest figures published by local organisation Miningsafety.co.za.

Doornkop is a single-shaft mine, which operates up to two kilometres down on the Kimberley and South Reefs. The mine produced 3,631 kilogrammes of gold in 2013.

South Africa’s gold mining has steadily decreased over the past 40 years, sliding from top global producer to world number six.

The country produced 167,235 kilogrammes of gold in 2012.