Russian forestry officials confirmed on Wednesday that areas contaminated by the 1986 Chernobyl disaster have been more widely affected by the current wildfires than authorities have admitted, Interfax news agency reported.
The heavily contaminated Bryansk region alone has seen 28 fires covering an area of 269 hectares that have since been extinguished, the officials said.
Russia's Surgeon General, Gennady Onishchenko, and local civil defence authorities had denied as recently as Tuesday that there were any fires in Bryansk.
The environmental protection organisation Greenpeace had called on Russian authorities to tell the truth about the extent of the radioactive danger, accusing them of downplaying the threat.
"The situation is difficult, but stable and controllable," the head of the Bryansk forestry department, Vladimir Kotenkov, was quoted as saying by Interfax on Wednesday.
There are no new fires in the region for now, he said.
But experts have warned that the blazes, winds and firefighting efforts could swirl up contaminated particles.
"Under particular circumstances with strong winds, the particles can fly all the way to Moscow and Eastern Europe," the ecologist Alexey Yablokov, of the Russian Academy of Sciences, said.
Meteorologists noted that the weather conditions are calm for now. No elevated pollutant measurements have been discovered in Moscow, according to non-governmental organisations.
But people living in Bryansk, which borders Ukraine and Belarus, have been concerned for days about their health.
"There are maps on which the radioactive contamination can be seen and there are maps on which the fires can be seen," the forestry officials wrote. "When you lay these maps over one another, it is clear to everybody that it is burning in the radioactive areas."
Other regions exposed to radioactive radiation have also seen blazes, including the area around Chelyabinsk in the Ural, which is home to several nuclear facilities, they said.
Firefighters have yet to get the country's devastating blazes under control. Although they extinguished 300 fires over the previous day, 290 new ones flared up, the Civil Defence Ministry was quoted as saying by Interfax Wednesday morning.
A record heatwave with temperatures far over 30 degrees Celsius has gripped the country for more than two months, without any significant rainfall.
The Civil Defence Ministry announced that firefighting planes were deployed once again, including in the areas surrounding nuclear facilities in Sarov and Snezhinsk.
The situation in the capital Moscow, meanwhile, eased somewhat when smog from nearby peat-bog fires cleared.
But scores of fires continued to burn in the area, leading meteorologists to warn that the situation could once again worsen in the coming days.
Russians are also starting to feel the economic impact of the drought and wildfires.
With crops destroyed, and logistics and supply lines affected by the out-of-control blazes, food prices have registered a spike of 15 to 20 per cent, according to the newspaper Rossiyskaya Gazeta.
That could lead inflation to increase beyond the 6 to 7 per cent expected by the government by year's end, it said.