MOSCOW — As if things in Russia were not looking sufficiently apocalyptic already, with 100-degree temperatures and noxious fumes rolling in from burning peat bogs and forests, there is growing alarm here that fires in regions coated with fallout from the Chernobyl nuclear disaster 24 years ago could now be emitting plumes of radioactive smoke.

Several fires have been documented in the contaminated areas of western Russia, including three heavily irradiated sites in the Bryansk region, the environmental group Greenpeace Russia said in a statement released Tuesday. Bryansk borders Belarus and Ukraine.

"Fires on these territories will without a doubt lead to an increase in radiation," said Vladimir Chuprov, head of the energy program at Greenpeace Russia. "The smoke will spread and the radioactive traces will spread. The amount depends upon the force of the wind."

Officials from Russia's federal forest protection service confirmed that fires were burning at contaminated sites on Tuesday, and expressed fears that lax oversight as a result of recent changes in the forestry service could increase the chances that radioactive smoke would waft into populated areas.

It is unclear what health risks the radiation could pose, or to what extent radioactive particles have spread in the weeks that wildfires have been raging throughout Russia, consuming villages and blanketing huge tracts with thick smoke.

The danger comes from radioactive residue still coating large areas of Ukraine, Belarus and Russia, years after the explosion of Reactor No. 4 at the Chernobyl nuclear power plant on April 26, 1986, in what was then the Soviet republic of Ukraine.

"The Chernobyl catastrophe occurred and these areas were littered with radioactive fallout," said Aleksandr Nikitin, director of the St. Petersburg office of Bellona, an international environmental group.

"This contaminated the trees and the grass." he said.

"Now, when there is a fire and when all of this burns, all of this radioactivity, together with smoke, comes out and spreads to other territories, including populated areas where people breathe it in as smog."

Russia's emergency minister, Sergei K. Shoigu, warned last week that the fires could release radioactive particles.

But with the government coming under criticism for its handling of the fires, which have left more than 50 dead and caused tens of millions of dollars in damage, little official information has been made available about the radioactive threat.

Responding to the Greenpeace statement on Tuesday, Dr. Gennadi G. Onishchenko, Russia's chief sanitary doctor, played down the danger.

"There is no need to sow panic," he told the Interfax news agency. "Everything is fine."

Dr. Onishchenko and other officials have already come under fire for appearing to cover up information on above-average mortality rates resulting from the high temperatures and heavy smoke. On Monday, Moscow's chief health official announced that the death rate had doubled in the capital because of the heat.

Russia has a history of whitewashing potentially embarrassing national disasters, a lingering legacy of the Soviet era. It took days for the Soviet government to inform its people of the Chernobyl explosion, leaving thousands unknowingly exposed to deadly radiation.

No one is saying that the radioactive fallout from the fires could reach the magnitude of the Chernobyl disaster. Scientists have known for years that fires in the contaminated zones have the potential to spread radioactive materials in small amounts.

The forest protection service has identified seven regions where dozens of fires have been burning in contaminated zones, with attention focusing on Bryansk, one of the regions most heavily contaminated by the Chernobyl disaster.