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Russian health officials are warning people in Moscow to stay inside and avoid physical exertion as smog from the worst wildfires in modern Russian history smothers the city.

Haze and smoke are spreading through Moscow's streets, even seeping into the Metro network, and some people are wearing face masks indoors.

Reports suggest the city's mortality rate rose by at least 30% in July.

The national death toll attributed directly to wildfires is 52.

Nearly 560 fires were still burning across central Russia as of Friday.

Of these, 39 were peat bog fires, 27 of them in the Moscow region alone, accounting for the acrid smoke choking the capital.

Daytime temperatures in Moscow remain close to 40C (104F) with little sign of relenting in the next few days.

The smog has been affecting the capital for a week, and appeared to have been easing - before it worsened on Friday.

Air traffic at two of Moscow's international airports has been disrupted.

In the south of Moscow, visibility at the international airport Vnukovo was reduced to 300m (yards), and at Domodedovo airport it was no more than 400m.

Seven regions are under a state of emergency. Nizhny Novgorod, Ryazan and Voronezh are reported to be the worst hit.

Up to 2,000 homes have been destroyed in the blazes, officials say.

Russia has announced it is banning the export of grain from 15 August to 31 December after drought and fires devastated about a fifth of its grain crop.

No data for Russia as a whole have been released on the effect of the heat and air pollution on seasonal mortality rates.

But a Moscow registry office told AFP news agency that the city's overall rate had risen by 50% in July compared with the same period last year.

"We recorded 14,340 deaths in Moscow in July, that is 4,824 deaths more than in July 2009," Yevgenia Smirnova told the agency.

"The increase started in July, as opposed to June when the figures were largely good. The heatwave has certainly had an influence."

The Russian news agency Interfax quoted an anonymous "informed" source as saying Moscow's mortality rate for July had risen by 29.7% directly as a result of the "catastrophic heat and smog".

When the agency's correspondents phoned the city's morgues to obtain a clearer picture of mortality rates, only a few responded, it said.

At one of them, the agency was told that the situation in July had been "twice as bad". At two others, medical staff were so busy, they had no time to speak to journalists.

Medical experts say the concentration of toxic particles in the air is far higher than the norm, and can be especially harmful to toddlers and the elderly.

Those vulnerable also include people with heart or respiratory problems.

The level of carbon monoxide in Moscow's air is more than three times higher than normal, officials say.

According to some experts, inhaling the polluted air is as dangerous as smoking several packets of cigarettes a day.