Brazilian President Jair Bolsonaro has authorised the use of the military to battle massive fires sweeping across parts of the Amazon as US President Donald Trump also offered his assistance.

Key points: US President Donald Trump has offered assistance to Brazil as massive blazes continue to rage through the Amazon

US President Donald Trump has offered assistance to Brazil as massive blazes continue to rage through the Amazon Brazilian President Jair Bolsonaro has authorised the use of the military to help battle the fires

Brazilian President Jair Bolsonaro has authorised the use of the military to help battle the fires French President Emmanuel Macron questioned Brazil's trustworthiness and threatened to block a trade deal

Brazilian forces will deploy starting on Saturday to border areas, indigenous territories and other affected regions in the Amazon to assist in putting out fires for a month, according to a presidential decree.

The armed forces will collaborate with public security and environmental protection agencies, the decree says. Mr Bolsonaro's office confirmed that he had signed it.

The decree comes as Mr Trump tweeted out that he stood ready to assist if needed.

So far this year, there have been nearly 77,000 wildfires across Brazil. ( AP: Leo Correa )

"Just spoke with President @JairBolsonaro of Brazil. Our future Trade prospects are very exciting and our relationship is strong, perhaps stronger than ever before. I told him if the United States can help with the Amazon Rainforest fires, we stand ready to assist!" Trump said in a post on Twitter.

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Mr Bolsonaro has previously described rainforest protections as an obstacle to economic development, sparring with critics who note that the Amazon produces vast amounts of oxygen and is considered crucial in efforts to contain global warming.

Charred trees and fallen branches were seen around Porto Velho, the capital of Rondonia state, which borders Bolivia.

In some instances, the burned fields were adjacent to intact livestock ranches and other farms, suggesting the fires had been managed as part of a land-clearing policy.

A large column of smoke billowed from one fire, and smoke rose from a couple of nearby wooded areas.

Fires in the Amazon have surged 83 per cent so far this year. ( AP: NASA )

Life appeared normal in Porto Velho. However, visibility from the windows of an arriving plane was poor because of smog enveloping the region.

Protests held across South America

Small numbers of demonstrators gathered outside Brazilian diplomatic missions in Paris, London, Geneva and Bogota, Colombia, to urge Brazil to do more to fight the fires.

Larger protests were held in Uruguay and Argentina. Hundreds also protested in Chile, Ecuador and Peru.

Demonstrations, like this one in Rio de Janeiro, took place around world. ( AP: Silvia Izquierdo )

Neighbouring Bolivia and Paraguay have also struggled to contain fires that swept through woods and fields and, in many cases, were set to clear land for farming.

About 7,500 square kilometres of land has been affected in Bolivia, according to Defence Minister Javier Zavaleta.

On Friday, a B747-400 SuperTanker arrived in Bolivia and began flying over devastated areas to help put out the fires and protect forests.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 2 m Wildfires in Brazil have destroyed vast swathes of the Amazon rainforest.

The US-based aircraft can carry nearly 76,000 litres of retardant, a substance used to stop fires.

Some 370 square kilometres have burned in northern Paraguay, near the borders with Brazil and Bolivia, said Joaquin Roa, a Paraguayan state emergency official.

He said the situation had stabilised.

Close to 20 per cent of the Amazon has already been deforested, according to Thomas Lovejoy, a George Mason University environmental scientist.

"I worry that the current deforestation will push past the tipping point leading to massive loss of forest and biodiversity," Mr Lovejoy said.

He said Brazil is "turning its back" on past environmental achievements, including the 1992 Earth Summit, and has proposed infrastructure projects that will accelerate the challenge of climate change.

"Fires are directly burning into the Amazon rainforest and that releases the carbon stored in those trees," NASA scientist Doug Morton said.

"The carbon then enters the atmosphere as carbon dioxide or methane, where it contributes to the greenhouse gases that are causing climate change, bringing us a warmer and a drier planet."

Mr Morton said there is now "an uptick in the pressure against the remaining Amazon forest, to expand agriculture production in areas that are the leading edge in the deforestation frontier".

Fires are common in Brazil in the annual dry season, but they are much more widespread this year.

Brazilian state experts reported nearly 77,000 wildfires across the country so far this year, up 85 per cent over the same period in 2018.

Just over half of those fires have occurred in the Amazon region.

Brazil contains about 60 per cent of the Amazon rainforest.

Brazilian forces will target fires at border areas, indigenous territories and other affected regions in the Amazon. ( AP: Victor R. Caivano )

France, Ireland threaten to block trade deal over fires

In escalating tension over the fires, France accused Mr Bolsonaro of having lied to French leader Emmanuel Macron and threatened to block a European Union trade deal with several South American states, including Brazil. Ireland joined in the threat.

The spectrum of possible economic repercussions for Brazil and its South American neighbours show how the Amazon is becoming a battleground between Mr Bolsonaro and Western governments alarmed that vast swathes of the region are going up in smoke on his watch.

Ahead of a Group of Seven summit in France this weekend, Mr Macron's office questioned Mr Bolsonaro's trustworthiness.

Brazilian statements and decisions indicate Mr Bolsonaro "has decided to not respect his commitments on the climate, nor to involve himself on the issue of biodiversity," Mr Macron's office said.

It added that France now opposes an EU trade deal "in its current state" with the Mercosur bloc of South American nations that includes Brazil, Argentina, Paraguay and Uruguay.

German Chancellor Angela Merkel views the fires as "shocking and threatening," according to government spokesman Steffen Seibert.

Argentina, which is struggling with rising poverty and austerity measures, has offered to send emergency workers to Brazil and Bolivia to help put out the fires. Chile has also offered aid.

The Brazilian Government has said European countries are exaggerating the country's environmental problems in order to disrupt its commercial interests.

Mr Bolsonaro, who has said he wants to convert land for cattle pastures and soybean farms, said it was difficult to curb increasing deforestation with limited resources.

"It's not easy to fight deforestation, our Amazon area is bigger than all of Europe," he said.

"We'll do what we can to fight this crime."

Reuter/AP