Emergency workers have been using heavy machinery to make firebreaks in the forest surrounding one of Russia’s top secret nuclear research centres.
Flames are already springing up in the woods around the Sarov facility – even though the closest blaze is still several kilometres away.
Officials say all hazardous materials have been removed from the centre as a precaution.
One engineer says the fires have been able to take hold because the government abolished the forest protection service four years ago.
Much of the area is peat bog – where fire can smoulder undetected for months.
Officials say around 600 seperate fires are still raging, mostly in western Russia, as the country swelters in the hottest summer on record.
Thousands of firefighters, troops and volunteers are doing all they can to contain the flames.
At first glance, smoke-logged Moscow resembles a city in the grip of a midwinter freeze.
The level of carbon monoxide here is six times greater than the safe limit.
Doctors have warned people are inhaling the equivalent of 40 cigarettes every few hours.