A major fire started in the southern Rostov region on Tuesday causing two days of explosions of World War II-era shells embedded in a local forest, a local spokeswoman for the emergencies ministry said.
"They are exploding where battles were fought," spokeswoman Marina Chernyavskaya told AFP.
She said no-one was hurt as a result of the blasts and more than 500 of emergencies personnel were proceeding with plans to put out the fire that now covered 200 hectares.
Across Russia, the emergencies ministry used 18 planes and 38 helicopters in an effort to douse a total of 220 wildfires, including 28 major blazes covering nearly 12,000 hectares, a Moscow-based spokeswoman told AFP.
With temperatures hovering well above 30 degrees Celsius, several commuters on the Moscow subway fainted due to the suffocating heat, national television said.
Muscovites have for days struggled with a heatwave although it has so far not been nearly as disastrous as the one recorded last year.
The country endured the worst heatwave in its recorded history in 2010, when peat bog fires left Moscow shrouded in smoke for several weeks, forcing people to quit the capital in droves and causing the city's mortality rates to double.
A record drought wiped out the harvest and wildfires spread out of control, killing dozens, burning down thousands of houses and threatening military and nuclear installations.
Environmental campaigners Greenpeace warned earlier this year that Russia could again choke in catastrophic wildfires this summer but no major signs of noxious smog have been detected around Moscow so far.