Brazilian President Jair Bolsonaro has authorised military operations in seven states to combat fires in the Amazon, responding to requests for assistance from their local governments, a spokeswoman for his office has said.

A video posted by the Brazilian defence ministry yesterday evening showed a military plane pumping thousands of litres of water out of two jets as it passed through clouds of smoke close to the forest canopy.

The response comes as leaders of countries in the G7 meeting in France expressed grave concerns over the fires.

French President Emmanuel Macron said the G7 was nearing a deal to provide "technical and financial help" to countries affected by the Amazon fires.

Nearly 80,000 fires have been registered across Brazil through 24 August, the highest since at least 2013, according to space research agency INPE.

Mr Bolsonaro announced the military would be sent in on Friday after several days of criticism from the public and world leaders that Brazil's government was not doing anything to fight the fires.

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But outside of Rondonia, the government had yet to give operational details for other states.

The defence ministry said in a briefing yesterday that 44,000 troops were available in Brazil's northern Amazon region, but did not say how many would be used where and what they would do.

Justice Minister Sergio Moro had also authorised a force of military police to assist in fighting the fires, with 30 set to be sent from Brasilia to Porto Velho.

The president's office posted to Twitter a photo of police officers on a plane bound for Rondonia set to arrive at noon.

Environment Minister Ricardo Salles posted a video showing a caravan of yellow fire prevention trucks and other government vehicles, saying they were on the ground responding in Rondonia.

The Amazon is the world's largest tropical rainforest and is seen as vital to the fight against climate change because of the vast amounts of carbon dioxide that it absorbs.

Brazilian climate scientist Carlos Nobre said he worries if 20-25% of the ecosystem is destroyed that the Amazon could reach a tipping point, after which it would enter a self-sustaining period of dieback as the forest converts to savannah.

Mr Nobre warned that it is not far off with 15-17% of the rainforest already destroyed.

Meanwhile, Bolivian firefighters continued to battle wildfires that burned more than 800,000 hectares of tropical forest in the province of Santa Cruz yesterday, with four helicopters, one Boeing SuperTanker and six planes deployed to extinguish the raging wildfires in the region.

Fires are threatening the livelihoods of the nearly 1.4 million indigenous people who depend on the tropical forest on the rim of the Amazon to make a living.

Video showed fires raging on the side of a highway in Otuquis National Park, near the border Bolivia shares with Paraguay and Brazil.

More than 646 new fires have been lit in the last 24 hours, according to the Bolivian forest and land authority.

Officials estimate that it will take 200 years for the area's forests to be restored to its original state.