Brazil's President Jair Bolsonaro has denied the existence of fires in the Amazon rainforest, calling it a "lie," despite data produced by his own government showing that thousands of fires are surging across the region.

Mr Bolsonaro last year similarly denied a spike in fires that provoked a global outcry.

The president's comments come even as Reuters witnesses in the remote Amazon town of Apui observed smoke blanketing the horizon in all directions during the day and large fires setting the sky aglow at night.

Fires in Brazil's Amazon for the month of August hit a nine-year high last year and this month so far looks even worse.

More than 10,000 fires have been recorded in the first ten days of August, up 17% from the same period a year ago, according to data from the country's national space research agency INPE.

But in a speech to other South American leaders, Mr Bolsonaro challenged foreign representatives to fly over the Amazon, saying that travelling by air from the far flung cities of Boa Vista to Manaus, you would not see a single flame.

"This story that the Amazon is going up in flames is a lie and we must combat it with true numbers," he said.

Mr Bolsonaro interfered in INPE after it released unfavourable data on Amazon deforestation last year, firing the agency's head Ricardo Galvao, who defended his agency's numbers that showed rising destruction.

In his speech, Mr Bolsonaro argued that Brazil has shown itself capable of protecting the Amazon because the majority of the forest is still standing.

He said the Amazon is a wet forest that preserves itself and does not catch fire.

The media and foreign governments are presenting a false narrative about the Amazon, he said.

Experts say that fires are not a natural phenomenon in the rainforest, but are usually man-made in order to clear deforested land for pasture.

Foreign pressure is mounting on Brazil to protect the world's largest rainforest, an ecosystem vital to combating climate change because of the vast amount of carbon dioxide that it absorbs.

Global investors managing more than $2 trillion have threatened to pull their investments out of Brazil's meat packers, grains traders and government bonds if Mr Bolsonaro's administration does not take action on Amazon destruction.