"I'm deeply concerned by the fires in the Amazon rainforest. 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."
The extent of the area damaged by fires has yet to be determined but smoke has choked Sao Paulo and several other Brazilian cities.
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.
But the WWF has blamed this year's sharp increase in accelerating deforestation in the Amazon, which is seen as crucial to keeping climate change in check.
"Even out here in the middle of the Atlantic Ocean, I hear about the record amount of devastating fires in the Amazon," tweeted teenage climate activist Greta Thunberg, who is on yacht voyage to New York.
"My thoughts are with those affected. Our war against nature must end."
The 16-year-old heads to UN climate talks in September before travelling to Canada, Mexico and then Chile for another UN conference in December.
Government 'lacks resources'
Brazilian President Jair Bolsonaro said on Thursday that the government lacks the resources to fight wildfires in the Amazon rainforest after satellite images showed a record number of burning spots this year.
In a speech broadcast live on Facebook, the president said the government is investigating the fires.
He also said he never claimed non-governmental organizations were starting fires in the forest, saying he was only talking of his suspicions.
Fires surge as deforestation accelerates
The number of forest fires in Brazil surged in the first eight months of 2019, official data show, as Mr Bolsonaro faces growing criticism over rampant destruction of the Amazon.
Nearly 73,000 fires were recorded between January and August, compared with 39,759 in the first eight months of 2018, the embattled National Institute for Space Research (INPE) said late Monday.
That is the highest number of forest fires for any year since 2013 and follows two years of declines.
"What we are seeing is a consequence of the increase in deforestation seen in recent figures," said Ricardo Mello of WWF's Amazon Program.
Norway on Thursday joined Germany in halting Amazon protection subsidies, accusing Brazil of turning its back on the fight against deforestation.
The governors of nine states spanning the Amazon also published a statement on Sunday saying they would negotiate directly with the Amazon Fund contributors.
The latest INPE figures coincide with a UN regional meeting on climate change in Brazil ahead of December's summit in Chile.
INPE is already in Mr Bolsonaro's crosshairs over data showing a surge in deforestation in recent months.
Mr Bolsonaro dismissed the figures as lies and sacked the head of the agency tasked with tracking forest clearing.
Brazil leads the region in forest fires this year, according to the INPE data that is collected via satellite and updated in real-time.
Venezuela ranked second with 26,453 fires and Bolivia with 16,101.
