Three people were killed on Tuesday in an accident at Norilsk Nickel's Taimyr underground mine in Siberia, the Russian mining giant said.
The world's largest palladium producer and second-largest nickel producer said the facility was still working after the incident. It is one of the company's main mines in Siberia and contains mixed ores.
At 16:30 local time three workers were found dead at a depth of 1,345 meters (0.8 miles), Nornickel said, adding that there had not been a fire, or a rockslide.
Nornickel, part-owned by Russian businessman Vladimir Potanin and aluminium giant Rusal, is also a major producer of platinum and copper.
Shares in Nornickel fell quickly in Moscow when the company reported the accident but recovered after it clarified that the mine was unaffected. The shares were up 0.6% as of 14:13 GMT.
The deaths could have been caused by methane gas poisoning, the state-run TASS news agency reported, citing an unnamed source.
"Three people died, no one is injured," TASS quoted the deputy prime minister of the Krasnoaysk region, Anatoly Tsykalov, as saying. "The reasons for the situation are being investigated. There were no technological violations at the mine."
According to Tsykalov, two miners were initially missed at the mine, with a third disappearing after going in search of them.
All three were found dead with unopened self-protective breathing equipment, which is supposed to be used in case of emergency, on them, he added.
"Nornickel is conducting an internal investigation into the incident and will provide new information as it becomes available," the company said in a statement.
The mine, launched in 1982, is one of the deepest in Europe. In 2015 Nornickel said the Taimyr facility produced 35% of the company's total nickel output, 21% of its copper, 38% of its cobalt and 15% of its platinum group metals.