"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 on 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 traveling to Canada, Mexico and then Chile for another UN conference in December.