Russian Prime Minister Dmitry Medvedev has ordered police to investigate if bushfires currently burning across more than 3 million hectares in Siberia were started deliberately to conceal illegal logging activity.

Key points: Environmentalists have dubbed the fires an ecological catastrophe

Environmentalists have dubbed the fires an ecological catastrophe Ten army transport planes and 10 helicopters have been sent to battle the fires

Ten army transport planes and 10 helicopters have been sent to battle the fires Donald Trump offered Vladimir Putin US help in putting out the blaze

Russia's Investigative Committee said it had opened a criminal case over the fires and was investigating officials in the Siberian city of Krasnoyarsk for suspected negligence in not taking action to stop them.

The bushfires cover 3.1 million hectares of mostly remote, uninhabited forest, the Federal Forestry Agency said on Thursday.

In July, scientists released alarming satellite images showing vast bushfires spreading across the Arctic Circle, with potentially dire consequences for the environment.

The fires are raging as sweltering heatwaves plague Europe and the US, and come after the planet experienced its hottest June on record.

Greenpeace activist Anton Beneslavsky said the Siberia fires were caused by a combination of climate change and what he said were ineffective control measures.

Local authorities have blamed a lightning strike.

Environmentalists have dubbed the fires an ecological catastrophe, and a petition circulated online by an ecologist from Siberia has attracted more than 870,000 signatures, demanding authorities take tougher action.

The Defence Ministry said 10 army transport planes and 10 helicopters had arrived in Krasnoyarsk, a day after President Vladimir Putin ordered the army to help firefighters battle the fires.

Smoke reaches Alaska, Canada and Russian cities

A NASA satellite image shows winds carrying huge plumes of smoke from multiple forest fires in southern Siberia. ( AP: NASA )

Firefighters have been battling forest fires across 116,000 hectares in Siberia's Irkutsk and several other regions, but other fires covering a total of 2.96 million hectares are only being monitored, the Federal Forestry Agency said.

NASA said in a statement on Wednesday its satellite imagery showed the smoke from the fires, which collectively cover an area the size of Belgium, had wafted across Russia's Siberia and Far East all the way to Alaska and down the west coast of Canada.

More than 800 inhabited settlements, from villages to cities, have been shrouded in smoke in the regions of Krasnoyarsk, Yakutia and Irkutsk, TASS news agency cited the emergency services as saying.

Separate online satellite imagery from fires.ru, a site run by a satellite monitoring company, showed that the fires appeared to be near the Kuyumba oilfield in eastern Siberia.

Russian oil giant Rosneft, which operates the field, declined to comment, but a source at the company said Rosneft had taken all possible measures to ensure that its facilities were safe.

Trump offers Putin help

The Kremlin said the two leaders had spoken by phone at Washington's initiative. ( AP: Evan Vucci )

In a rare phone call late on Wednesday, US President Donald Trump offered his Russian counterpart Vladimir Putin help in putting out the fires.

Mr Putin told Mr Trump that Moscow would take him up on his offer if needed.

The Kremlin said the two leaders had spoken by phone at Washington's initiative.

"President Putin expressed his sincere gratitude for such an attentive attitude and for the offer of help and support," the Kremlin said in a statement.

"The Russian President took this step from the US President as a sign that in the future we can restore full-scale ties between our two countries."

US-Russia relations have been strained by allegations of Russian interference in US politics, which Moscow denies.

Reuters