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 3,000 kilometers (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 searching the tunnels have not established contact with any of the missing.
Full ventilation has not been restored to the mine and rescuers are forced to work with oxygen masks.
Those missing are believed to be some 500 meters (1,600 feet) below the surface and about 1.5 kilometers (nearly a mile) from the nearest shaft, Kemerovo governor Aman Tuleyev said.
"By the time the rescuers get to the main collapse area, their oxygen is almost at it 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.
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.
Putin ordered officials to investigate "how production technology was observed, how control instruments operated, what measures the mine managers took to raise reliability, what was the state of individual means of protection and how rescue operations were organized," the ITAR-Tass news agency reported.
There was no information on what set off the blast. Mine explosions and other industrial accidents are common in Russia and other former Soviet republics, and are often blamed on inadequate implementation of safety precautions by companies or by workers themselves.
But Tuleyev was quoted by the business newspaper Vedomosti as saying the mine was one of the world's most technically advanced.
The deadliest explosion in Russia's coal mines in decades occurred in March 2007, when 110 miners were killed.
The Raspadskaya mine produces about 10 percent of Russia's coking coal, Vedomosti said, and a long interruption of production could affect Russia's steel industry.
Associated Press Writer Jim Heintz in Moscow contributed to this report.