MEZHDURECHENSK, Russia — Thirty-eight Siberian coal miners are buried so deep in Russia’s largest underground coal mine that rescuers use up most of their oxygen tanks trying to reach them and can’t spend much time searching for the missing men, the regional governor said Tuesday.

Prime Minister Vladimir Putin traveled to the Raspadskaya mine, about 1,900 miles east of Moscow, to observe rescue operations, and he raised a series of sharp questions about mine safety and whether the initial rescue work was conducted improperly.

The death toll from the two explosions that hit the mine in the Siberian region of Kemerovo rose to 52, and prospects of finding any survivors nearly three days after the blasts were dimming. Emergencies Ministry spokeswoman Veronika Smolskaya said rescuers have not established contact with any of the missing miners.

Ventilation has not been restored to the mine and rescuers are forced to work with oxygen masks.

Those missing are believed to be 1,600 feet below the surface and nearly a mile from the nearest shaft, Kemerovo Gov. Aman Tuleyev said.

“the time the rescuers get to the main collapse area, their oxygen is almost at its limits. Therefore, little time remains to clean up the collapse,” he said, according to the ITAR-Tass news agency.

He said two ventilator shafts are ready to be put in operation, but that there are points where the coal is smoldering, raising the risk that pumping in additional air could set off a new blast.

Emergencies Minister Sergei Shoigu said that ventilation would be restarted today, but not at full power and only in parts of the main shaft because methane was still accumulating in some of the tunnels.

Many of the dead were rescue workers who went into the shafts after the first blast late Saturday and were caught in the second explosion, which was so powerful that it shattered the main shaft and a five-story building at the mine head.

Both the explosions are being blamed on methane, and Putin questioned why rescuers were sent into the mine without a preliminary assessment of the gas concentration, according to the RIA Novosti news agency.

The head of the mine rescue service, Alexander Sin, said rescuers are under orders to immediately render help, the agency reported.