The death toll in Russia's worst mining disaster in years climbed to 36 on Sunday as officials said 26 workers missing following methane explosions at the pit in the country's frozen Arctic north could not have survived.

Two blasts ripped through the Severnaya coal mine on Thursday at a depth of 748 metres, killing four miners and trapping 26 others.

Six more people were killed on Sunday as a fresh explosion hit the mine in the city of Vorkuta in the Komi region, once home to one of the most feared Soviet-era Gulag labour camps.

"According to the expert technical council, 26 (missing) people who were in the mine had no chances of surviving," Tatyana Bushkova, a spokeswoman for the mine's operator Vorkutaugol, told AFP on Sunday.

"The rescue operation has been halted," she added in an emailed statement.

Anton Kovalishin, a spokesman for the emergencies ministry in the Komi region, told AFP a new explosion in the early hours of Sunday killed five rescue workers and a miner.

"This is a difficult emergency situation, a difficult catastrophe for Russia, for our mining industry," said Deputy Prime Minister Arkady Dvorkovich, who is heading a special commission established to deal with the tragedy.

He said the families of the victims would each receive 1mn roubles ($13,000) in compensation.

Vorkutaugol is operated by Severstal, the Russian steelmaker controlled by billionaire Alexey Mordashov.

The company was considering whether to flood the mine but now apparently decided to pump it with nitrogen to stop the fire burning at the pit, said Bushkova.

"The chairman of Severstal's board of directors Alexei Mordashov said that he expects the Severnaya mine to continue its work after the consequences of the disaster have been liquidated," she said.

"This is the company's largest mine that produces about a fourth of Vorkutaugol's output."

Authorities launched a massive search operation involving hundreds of rescue workers who had been trying to track down the missing despite almost zero visibility, smoke, gas-polluted air and rubble.

Both the company and the authorities had until now refused to declare the missing dead even though rescuers had apparently failed to make contact with them over the past few days.

But the latest explosion forced officials to admit that no one could have survived.

"Unfortunately, we are forced to acknowledge that all the conditions at that section of the mine would not allow a person to survive," Emergencies Minister Vladimir Puchkov said in comments broadcast by LifeNews television channel.

'Risk of new explosions'  

Seventy-seven people were in the mine during the rescue operation when the new explosion hit on Sunday, the emergencies ministry said.

Of these, 71 were rescued and brought to the surface. Eleven of them were injured.

"According to experts, there is a high risk of new explosions," the ministry said.

Sunday, Monday and Tuesday have been declared days of mourning in the region.

"We are unable to bring people from the dead but we will do everything to support their families at this difficult hour," acting head of the Komi region, Sergei Gaplikov, told families of the victims, in remarks released by his office.

Earlier this week President Vladimir Putin tasked the government with setting up a special panel to look into the accident.

The Investigative Committee has opened a criminal probe into the violation of safety rules and dispatched investigators and forensic experts to the scene.

Mine accidents are fairly common in Russia and other former Soviet countries, where much of the infrastructure has not been modernised since the Communist era.

The explosions at the Severnaya mine took place despite the fact that the company has over the past years invested heavily in safety, Vorkutaugol said.

"We are constantly spending lots of money - and we will spend it in the future - to perfect the technical systems that ensure safety and prevent violations," Mordashov was quoted as saying.