Catastrophic forest fires — spurred by hot and dry weather — have ravaged vast areas of Siberia, and ruined the region’s ecosystem, Russian ecologists said.

Fires have destroyed 100,000 square kilometres of forests across Russia — an area bigger than Bihar — since the start of the summer season this year, according to Russia’s Greenpeace.

The situation is worst in central Siberia, where fires have ravaged 50,000 square kilometres of forests.

Russian authorities claim the ecologists’ estimates are greatly overblown. The Emergencies Ministry said the fires have so far covered an area of 6,530 square kilometres.

However, Greenpeace maintains that its figures are far more accurate as they are based on satellite imagery.

Thick smoke has enveloped Novosibirsk, Tomsk, Omsk and other major cities in Siberia, closing down local airports for days. Plumes of smoke rising high into the atmosphere have been carried by winds as far across the Pacific as British Columbia in Canada.

More than 6,000 fire-fighters; a dozen aircraft; and hundreds of volunteers are fighting the fires in Siberia, but have so far failed to contain the calamity.

“The forest fires are so vast that rains alone can stop them,” said Alexei Yaroshenko, who heads the forest section in Greenpeace Russia. “This year may beat all previous records for the scale of wildfires.”

While authorities blame the fires on the hottest and driest summer in Siberia in more than 40 years, ecologists fault a reform of the forestry service six years ago, when the government drastically downsized the forestry manpower and cut financing.

According to Mr. Yaroshenko, the forests destroyed by wildfires every year in Russia are so big that cannot be replanted.

“The fires have altered the taiga ecosystem in some areas of Siberia and the Far East, transforming it into ‘green deserts’ overgrown with grass, which provokes more fires when it gets dry,” the ecologist said.