At least 11 miners died in the initial accident, when coal dust ignited in a ventilation shaft in the Listvyazhnaya mine in the snowbound Kemerovo region, filling the mine with smoke, the TASS news agency cited local emergency services as saying.
It quoted the acting head of the Emergencies Ministry as saying three rescuers had also been killed.
With methane levels in the mine dangerously high, an operation to rescue 35 people stuck underground had to be halted, regional governor Sergei Tsivilev said.
Three rescuers were also reported missing.
"The chance of an explosion is very high. We've decided to suspend the search and rescue operation until the concentration of gas reduces," Tsivilev said.
Dozens of people were being treated in hospital, at least some of them for smoke poisoning.
Four were in critical condition.
The coal-producing region of Kemerovo, roughly 3500km east of Moscow, has suffered fatal mining accidents for years.
The Listvyazhnaya mine is part of SDS-Holding, owned by the privately held Siberian Business Union.
The owner had no immediate comment.
Before the rescue operation was halted, the governor said there was still electricity and ventilation in the mine but that contact had been lost with some of those deep underground.
"For now there is no heavy smoke, so we hope that there is no fire," Tsivilev said on social media.
"We have no communication lines with these people, the underground communications system is not working."
The Emergencies Ministry said 285 people were inside the mine when smoke spread through the ventilation shaft.
Authorities said at least 239 had made it to the surface.
On state television, President Vladimir Putin said he had spoken with the governor and emergency officials.
"Unfortunately the situation is not getting easier. And there's a danger to the lives of the rescuers," he said.
"We'll hope they can save as many people as they can."
Kemerovo declared a three-day period of mourning.
14°C