The Kremlin's palaces and churches are seen through the smog blanketing Moscow from wildfires in the countryside on Wednesday. (Alexander Zemlianichenko/Associated Press)

Wildfires that have wiped out Russian forests, villages and a military base sent the thickest blanket of smog yet over Moscow on Wednesday.

Passengers on Moscow's subway said an eye-stinging haze was hovering above platforms, as city hall warned of health risks from the smoke carrying harmful gases including carbon monoxide.

To the east, firefighters focused on beating flames back from a nuclear weapons research facility, while in Moscow President Dmitry Medvedev fired several high-ranking military officials over what he called criminal negligence in fires that ravaged a military base.

Russia is suffering its worst heat wave on record, helping to ignite forest and bog fires across central and western regions.

Over the last 24 hours, firefighters have extinguished 293 fires, but another 403 have been spotted while more than 500 continued to rage over large swathes of countryside, some of them out of control, the Emergencies Ministry said.

The fires have killed at least 48 people in the past week.

Dry winds have sent clouds of smog over Moscow, but Wednesday's was the thickest yet, with the haze obscuring the capital's landmarks and penetrating the subway system.

Moscow's 10 million residents were cautioned to guard themselves against the polluting smog, which comes from fires of peat bogs to the south and east of the city. The bogs were drained in Soviet times to harvest peat, leaving them prone to wildfires — especially in heat waves.

Pollution levels 'critical'

Pollution indicators in the capital reached a "critical barrier" overnight, and "even healthy people must take preventive measures," Moscow weather officials said in a statement. It did not say what action should be taken, but officials have urged people to wear face masks outdoors.

"I woke up before dawn and thought I was going to die of suffocation," said Yadviga Pashkova, a frail, 62-year-old former schoolteacher who lives in central Moscow. "It felt awful because there was no way out."

Russian Prime Minister Vladimir Putin, centre right, speaks with people whose homes were destroyed by forest fires in Russia's Voronezh region, on Wednesday. (Alexei Nikolsky/Pool/Associated Press)

The smoke and smog in Moscow also have sickened and killed wild and pet birds, who are especially sensitive to air pollution, said Vladimir Romanov, director of the Green Parrot Hospital. He could not give figures, saying no records are kept and his facility sees only a fraction of the cases.

Some 400 kilometres to the east, about 2,000 army troops and emergency personnel were fighting back flames that surrounded Russia's top nuclear research facility in Sarov.

The situation there was "tense but not critical, Deputy Defence Minister Dmitry Bulgakov said, after new robotic firefighting equipment was sent to the scene overnight.

"There is no threat to the Federal Nuclear Centre, and there is no reason for worry," Bulgakov was quoted by the ITAR-Tass news agency as saying. Tass quoted the country's nuclear chief, Sergei Kiriyenko, as saying all explosive and radioactive material has been moved off site as a precaution.

The top-secret facility is Russia's main nuclear research centre, and the birthplace of Soviet nuclear weapons. Lawyers for the late Russian security agent Alexander Litvinenko, who died in London in 2006 after drinking tea laced with polonium, claimed the radioactive isotope was produced at Sarov.

Military brass fired

Medvedev, meanwhile, fired the chief of Russia's naval aviation and at least seven high-ranking military officials a day after officials confirmed the fires burned at least half of the buildings at a military base near Moscow containing unspecified aviation equipment. Russian media said up to 200 naval aircraft may have been destroyed.

"If something similar happens in other places, in other agencies, I'll do exactly the same thing, with no sympathy," Medvedev said at a security council meeting Wednesday.

The weather this week will not likely help the firefighting efforts, as temperatures in Moscow and to the south and east were forecast to reach 38 C.