Acrid smoke from forest and peat bog fires which blanketed Moscow until early last week returned to the Russian capital Sunday as wildfires are still burning around the city.

The concentration of carbon monoxide in Moscow air on Sunday morning was more than five times higher than the usual level as shifting winds brought the suffocating smog back from central Russia where three major peat bogs were burning, local media reported, citing officials.

City authorities once again called on residents to close windows and wear respiratory masks due to the return of the poisonous smog, which has been blamed for the mortality rate rise in Moscow.

Tens of thousands of Muscovites were forced to leave the city, while others complained about headaches and nausea. Health experts say even healthy people may feel sick in these unfriendly conditions.

The Civil Defense Ministry said Sunday that across Russia there were now 498 out-of-control fires covering an area of 53,000 hectares of land, down from over 80,000 hectares on Friday.

But blazes still rage in a wildlife reserve near Russia's top nuclear research facilities in Sarov, a small town 300 km east of Moscow, officials warned.

"The fire threatens Sarov as long as the Mordovsky (wildlife reserve) is burning," said Sergei Kiriyenko, head of Russian state nuclear energy corporation Rosatom.

However, 3,000 firefighters operating in Sarov made sure that the peril is no longer critical, he was cited by RIA Novosti news agency as saying.

A week ago, amid abnormal high temperatures toxic smoke crept into every corner of Moscow, even underground into the metro, forcing Muscovites to flee en masse.

On Aug. 9, a statement published on the Meteonovosti weather forecast website said "people who suffer from heart or lung problems, diabetes, thyroid gland conditions or metabolic disease are dealing with (the ecological situation) extremely hard due to lack of oxygen in the air."

A record number of more than 100,000 passengers have departed Moscow in one day, local media reported on Aug. 9, adding that the mortality rate in Russia's capital has doubled with the record-breaking heat and toxic smoke smothering the city.

"In common days, some 360 to 380 people die daily. Now, the number of deaths has increased to about 700. The mortality rate has doubled," Andrei Seltsovsky, head of the city's Health Protection Department, was quoted by RIA Novosti news agency as saying.

Amid the worst heatwave in 1,000 years of its recorded history, Russia has been battling the wildfires over the past month, which had killed over 50 people and left thousands homeless.

The unprecedented high summer temperatures have also destroyed at least 10 million hectares of crops, or 20 percent of Russia's cultivated land in total, leading to a ban on its grain export until the end of the year.