A fire at a St Petersburg hospital has killed five coronavirus patients in an intensive care unit.
The blaze was apparently started by a short-circuit in a ventilator, Russian news agencies reported.
The fire was quickly put out and 150 people were evacuated from the hospital, the country's emergency ministry said. It is not clear how many people have been injured.
All the patients who died at St George Hospital had been on ventilators.
"The ventilators are working to their limits. Preliminary indications are that it was overloaded and caught fire, and that was the cause," a source at St Petersburg emergencies department told the Interfax news agency.
Russia's NTV news website reports that the fire did not spread beyond one small Covid-19 ward on the sixth floor.
It quotes doctors as saying a short-circuit caused a ventilator "literally to explode" because of the oxygen concentration, and the ward filled with smoke, which suffocated the patients.
There have been persistent reports of a shortage of ventilators in Russia, especially in the provinces - as President Vladimir Putin himself acknowledged last month, the BBC's Sarah Rainsford reports.
Production has increased rapidly, but research by the Reuters news agency found that outside Moscow many ventilators are old - made in the 1990s.
Doctors working at full stretch
St Petersburg, with a population of approximately 4.9 million people, has 5,483 hospital beds for Covid-19 patients.
The city's hospitals have been overstretched by the outbreak and doctors are working at full capacity, says BBC Russian reporter Anna Pushkarskaya.
St George's was seen as less affected than others, but it was quarantined last month when 18 patients were diagnosed with the virus.
A police source quoted by Tass news agency said the ventilator which caught fire was new - it had been installed just this month - and was made by Russia's Ural machine-building plant.
St Petersburg has recorded just over 8,000 cases of Covid-19 so far - far fewer than Moscow, where the infection rate is continuing to climb at over 10,000 new cases daily.
State investigators have opened a case to determine whether there was criminal negligence - either in the ventilator design and manufacture or in the hospital's fire precautions.
The All-Russia Institute for Medical Technology Research points out that there are many different types of medical ventilator, so a fault in one may not be common to others.
Alexei Kurinny, a member of the Russian parliament's health committee, said it was unlikely that a ventilator could have short-circuited or overloaded, and that fire safety was built into their design.
The St George Hospital in the Vyborg district had been converted to a Covid-19 hospital at the end of March.
The emergency services sent 105 firefighters and 55 vehicles to the hospital, offficials said.
The news of the fire comes as the country is starting to ease lockdown restrictions. Construction, farming and factory workers are resuming their duties.
Russia now has the second-highest number of confirmed infections worldwide after the United States. On Tuesday, it reported another 10,899 infections in the past 24 hours, bringing the total to more than 232,000.
The capital, Moscow, is the worst-affected area and has reported more than 5,000 new cases in the past 24 hours.