Several regions in Siberia are battling massive wildfires that have left cities shrouded in smog after a heatwave, an environmental group said on Monday.

"More and more burns every year," said the head of the forest programme at Russian Greenpeace Alexei Yaroshenko. "Last year nine million hectares (22 million acres) burned in the whole year, this year 10 million has burned already."

The full scope of the damage was hard to estimate, he said, partly because the areas worst affected are covered by thick haze that makes satellite monitoring difficult, but 2012 may set a record for the past decade.

A string of flights were cancelled at airports in the Siberian cities of Omsk and Tomsk at the weekend due to the smog, forcing passengers to be bussed to other cities.

Russia's emergency ministry played down the fires, with its Tomsk branch issuing a statement Monday that focused on the amount of water dropped from airplanes in an effort to douse the flames.

It said there was no risk to towns but warned that "the entire Tomsk region may be covered by smoke and ash" as the dry spell continues. The region went through several days of temperatures in the high 30s earlier this month.

The central emergency ministry website said Sunday that 17,000 hectares continued to burn in Siberia.

Yaroshenko however put the affected area at at least two million hectares, adding that the situation got out of control because regional officials always hide the scope of the problem from their Moscow superiors.

"It is very far from Moscow and impacts only a few percent of the population, so it is kept quiet," he told AFP. "They began to rally for help when the situation was out of control."

"The main method of wildfire suppression continues to be lying," he said.

Russian wildfires made international headlines in 2010 when they left Moscow shrouded in smoke for several weeks. The fires killed at least 60 people and destroyed over 3,000 homes.

At the time, then president Dmitry Medvedev warned he would make Russian officials fight fires with their own hands if they fail to prevent them again.