Official figures show nearly 73,000 forest fires were recorded in Brazil in the first eight months of the year - the highest number for any year since 2013. Most were in the Amazon.
The extent of the area damaged by fires has yet to be determined, but smoke has choked Sao Paulo and several other Brazilian cities in the past week.
UN chief Antonio Guterres said he was "deeply concerned" by the fires.
"In the midst of the global climate crisis, we cannot afford more damage to a major source of oxygen and biodiversity," he said on Twitter.
"The Amazon must be protected."
Amazon burning: Brazil's president blames NGOs for forest fires
France's President Emmanuel Macron said the wildfires were "an international crisis" and called on the globe's most industrialized nations to address it at their summit this weekend.
"Our house is on fire. Literally. The Amazon, the lung of our planet which produces 20 per cent of our oxygen is burning," Macron said on Twitter.
"It is an international crisis. Members of the G7, let's talk in two days about this emergency."
That did not sit well with Brazil's President Jair Bolsonaro.
"The French president's suggestion that Amazon issues be discussed at the G-7 without participation by the countries in the region evokes a colonialist mentality that is out of place in the 21st century," Bolsonaro wrote on Twitter.
Neighboring Peru, which contains much of the Amazon basin, announced it was "on alert" for wildfires spreading from the rainforest in Brazil and Bolivia.
Paraguay and Bolivia are battling separate wildfires that have devastated large areas of their rainforests.
Forest fires in Brazil surge as deforestation accelerates
Environmental specialists say the fires have accompanied a rapid rate of deforestation in the Amazon region, which in July quadrupled compared to the same month in 2018, according to data from the National Institute for Space Research (INPE).
Bolsonaro instead attributes the fires to increased drought and accuses environmental groups and NGOs of whipping up an "environmental psychosis" to harm Brazil's economic interests.
"This environmental psychosis lets you do nothing," the president lamented, adding that it was hampering the country's development.
"I don't want to finish the environment, I want to save Brazil," said Bolsonaro, a climate change sceptic who had advocated opening up tribal lands and protected areas to farming and mining interests since assuming office in January.
Bolsonaro's comments come as Brazil hosts a UN regional meeting on climate change in the northeastern city of Salvador ahead of December's summit in Chile.
Amazon fires
A senior Brazilian official defended Brazil's environmental policy at the conference and said it complied with the Paris Agreement against global warming.
"We are teaching the world how to produce. In only 29 per cent of our territory, we produce food for everyone.
"Worldwide, the average land use for agriculture exceeds 50 per cent - we only use 29 per cent," said Roberto Castelo, an environment ministry official who was roundly booed by greens at the conference.
"I do not defend the burnings, because there always was and always will be burnings. Unfortunately, this has always happened in the Amazon," Bolsonaro said, referring to the dry season, land-clearing fires.
"But accusing me of being a Captain Nero setting fire to things is irresponsible. It is campaigning against Brazil," the president told reporters outside his Brasilia residence.
The reference to Captain Nero appeared to be to the Roman emperor said to have fiddled while Rome burned. Bolsonaro is a former army captain.
Forest fires tend to intensify during the dry season, which usually ends in late October or early November, as land is cleared to make way for crops or grazing.
Celebrities slam media for ignoring Amazon rain forest fire
"Just think, if the world begins imposing trade barriers, our agribusiness will fall, we will start to go backwards, the economy will start to get worse - your life, the lives of newspaper editors, television owners, the lives of all Brazilians will be complicated, without exception. The press is committing suicide," Bolsonaro said.
However, there are signs of growing concern from within the powerful agribusiness sector over Bolsonaro's environmental isolationism.
The governors of Brazil's Amazon states have also criticized the government for recent decisions by Germany and Norway to suspend Amazon aid projects.
"This week two big German media outlets expressed the idea that it was time to start boycotting Brazilian products. It's only a matter of time," Marcello Brito, head of the Brazilian Agribusiness Association, told the Valor daily.
"The question is, who is interested in transforming Brazil into an environmental pariah," he asked.
"We cannot change the president of the republic. What our sector can do is work, in a unified way, to try to reverse the damage as much as possible."
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