Four miners were killed and 26 were missing after a pit collapse in northern Russia, in the latest tragedy to hit the accident-prone industry, investigators said on Friday.
A total of 110 people were on duty at the Severnaya coal mine in the remote Komi region on Thursday when it was hit by a "pressure burst", Vorkutaugol, which owns the mine, said.
Eighty workers were rescued and brought to the surface.
A spokeswoman for Vorkutaugol, Tatyana Bushkova, said a methane explosion had also taken place in the mine.
"But so far it's difficult to say what was the root cause of the accident," Bushkova told AFP.
Vladimir Markin, a spokesman for the Moscow-based Investigative Committee, Russia's main investigative agency, said: "Currently the fate of 26 miners is unknown, four people died."
Hundreds of rescue workers were trying to trace the missing on Friday, the emergencies ministry said.
Bushkova said that all the places where the miners could be had been identified, adding that a fire was still burning at the site of the blast.
In a sign of the seriousness of the situation, Emergencies Minister Vladimir Puchkov flew to the Arctic region Thursday evening to personally oversee the rescue operation.
The Investigative Committee has opened a criminal probe into the violation of safety rules at the mine and dispatched a number of investigators and forensic experts to the scene.
Mine collapses are common in Russia and other former Soviet countries, where some infrastructure has not been modernised since the Communist era.