Russian President Vladimir Putin on Wednesday called in the army to fight forest fires that have been raging across vast expanses of Siberia for days, enveloping entire cities in black smoke.

Environmentalists have warned that the scale of the blazes could accelerate global warming, aside from any immediate effects on the health of inhabitants.

Around 3 million hectares of land in the center and east of the country were in the grip of fires on Wednesday, authorities said.

The acrid smoke has affected not only small settlements but also major cities in Western Siberia and the Altai region, as well as the Urals such as Chelyabinsk and Yekaterinburg, and disrupted air travel.

"After reviewing a report from the emergency situations minister, Putin instructed the ministry of defense to join the effort to extinguish the fires," the Kremlin's press service told Russian media.

Some 2,700 firefighters were already working to tackle the blazes, Interfax news agency reported.

The defense ministry told news agencies that 10 planes and 10 helicopters had been dispatched to the Krasnoyarsk region, one of the worst affected.

Russian Prime Minister Dmitry Medvedev promised help and insisted the fires represented no "immediate danger" for the population, during a visit to Krasnoyarsk on Wednesday.

"There are objective problems: The issue of distances, hard-to-reach regions and factors specific to this year," he said on live television.

The head of Russia's consumer safety watchdog, Anna Popova, meanwhile said the black smoke did not pose "­major risks" to people's health.

The Kremlin press service said the armed forces in the Irkutsk region, also badly hit, had been put on high alert, without providing further details of military involvement. 

The fires, triggered by dry thunderstorms in temperatures above 30 C, were spread by strong winds, Russia's federal forestry agency said.

States of emergency have been declared in five Russian regions. 

People living there have uploaded images to social media showing roads hazy with smoke and the sun barely visible in the sky.  

The majority of the fires, however, are raging in remote or inaccessible ­areas. Authorities make the decision to ­extinguish them only if the estimated damage exceeds the cost of the operation.