It was the second such deadly blaze in less than five months, underlining the widespread neglect of fire safety standards in Russia.
The fire in the one-story hospital in the village of Luka in the northwestern Novgorod region erupted around 3 a.m. Friday and quickly engulfed the structure, the Emergency Situations Ministry said.
The Investigative Committee said rescuers so far have recovered 10 bodies. It did not explain how it confirmed about the other deaths.
The agency added the blaze was apparently sparked by a patient.
State Rossiya 24 television reported that a witness said a smoking patient caused the fire. It said a nurse tried to put out the flames with a blanket but they spread quickly. The man who triggered the fire was saved, the station said.
Local prosecutors said the patient might have deliberately set his bed on fire.
Emergency officials had demanded the facility be closed after it failed a fire safety check earlier this year. The hospital administration, however, won permission to use it until next year.
Emergency officials said 23 of the 60 people in the building when the blaze broke out were evacuated. Emergency teams were searching the ruins for more bodies and combing a nearby forest for patients who may have fled the blaze or wandered off. Russian officials, however, said from the start they had little hope of finding any survivors.
The head of Russia's top state investigation agency flew to the area to personally oversee a probe.
Russia has a poor fire safety record with about 12,000 fire deaths reported in 2012. By comparison, the U.S., with a population roughly double Russia's, recorded around 3,000 fire deaths in 2011.
A fire at a psychiatric hospital near Moscow killed 38 people in April.
Russia's rights ombudsman Vladimir Lukin called Friday for civil society to re-establish control over the country's mental hospitals in light of the deadly fire in Luka.
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