Deaths in Moscow have doubled and are now averaging 700 people a day amid a sweltering heat wave and poisonous smog from wildfires.

Moscow health chief Andrei Seltsovky blamed weeks of unprecedented heat and suffocating smog for the rise in mortality compared to the same time last year.

He said city mortuaries are nearly overflowing, with 1,300 bodies, close to their capacity.

Acrid smog blanketed Moscow for a sixth straight day on Monday, with concentrations of carbon monoxide and other poisonous substances two or three times higher than what is considered safe.

Those airborne pollutants reached a record over the weekend - exceeding the safe limit by nearly seven times.

About 550 separate blazes were burning nationwide on Monday, mainly across western Russia, including about 40 around Moscow.