The leaders of the G7 club of rich countries meet in southern France on Saturday, with the burning Amazon, diving stock markets and their own stark divisions giving little grounds for optimism.

US President Donald Trump and fellow Western leaders will also face protests as they arrive in the famed surfing town of Biarritz -- though a heavy police presence will keep them far from view.

Thousands are set to march around 30 kilometres (20 miles) down the coast from Biarritz to denounce leaders over poverty and environmental damage.

The summit was already shaping up to be a difficult encounter with Western relations badly strained by Trump, but images of billowing smoke above the Amazon rainforest have lent it a new, even darker mood.

"The Amazon is burning and it's something that concerns everyone," Macron said on Friday in an interview with the Konbini website.

He has led international pressure on Brazilian President Jair Bolsonaro over the fires, telling him Paris would block efforts to seal a major trade deal between Latin America and the EU.

He has called for emergency talks at the G7, which lasts from Saturday to Monday, aiming for "concrete measures" to tackle the crisis.

"We are going to try and mobilise everyone to raise funding for reforestation as quickly as possible," Macron added on Friday.

Bolsonaro responded in televised remarks, saying "there are forest fires all over the world, and this cannot be used as a pretext for possible international sanctions," adding that "some countries" will defend Brazil at the G7 meet.

Talks in the beach resort, known for its fierce rainstorms that blow in from the Atlantic, will also be dominated by the darkening clouds over the world economy.

Wall Street stocks tanked on Friday after Trump escalated his trade war with China that is seen as responsible for a global slowdown.

"We don't need China and, frankly, would be far... better off without them," Trump tweeted on Friday, adding that US companies were "hereby ordered to immediately start looking for an alternative to China."

His outburst on Twitter came after China imposed tariffs on US imports worth $75 billion in response to an earlier round of American measures.

But Trump hit back immediately, raising tariffs still further.

"We see trade tensions as the single most important threat to global growth," a top EU official told reporters ahead of the G7 summit on condition of anonymity.

Trump continues to threaten European companies with trade tariffs, including Germany's car industry and France's wine sector.

Last month, he promised "substantial reciprocal action" after French MPs backed a new law imposing a sales taxes on US digital giants such as Google and Facebook.

"The president will raise the highly discriminatory digital services tax that France has decided upon," a US official said ahead of the summit that will see Trump and Macron hold talks.

Though the Amazon fires and trade are set to dominate the agenda, the G7 meeting will also be the full international debut of British Prime Minister Boris Johnson.

He will meet Trump for the first time as leader and is expected to discuss the UK's impending exit from the European Union, which the US president has enthusiastically backed.

"My message to G7 leaders this week is this: the Britain I lead will be an international, outward-looking, self-confident nation," he said on the eve of the summit.

But though Johnson needs Trump's support for a free-trade deal, he is at odds with him on a range of issues including the Iran nuclear crisis, climate change and global trade.

"Trade tensions are unsettling the global economy," a British official told reporters. "There are differences with the US about how to resolve global trade imbalances."

Trump will find himself under pressure from the Europeans, particularly Macron, to ease off on his policy of "maximum pressure" on Iran over its nuclear programme.

Since pulling out of the landmark 2015 nuclear agreement limiting Tehran's nuclear programme, Trump has slapped crippling sanctions on the Iranian economy.

Macron wants him to put a "pause" on the policy, an aide said recently, which would enable talks to take place to try to find a new diplomatic solution to the crisis.

Iranian Foreign Minister Mohammad Javad Zarif told AFP on Friday that Macron's "suggestions" to find a way out of the current impasse were "moving in the right direction."

A total of 13,000 French security forces are on duty around Biarritz to guard against violence, with authorities wary about anti-government "yellow vest" protesters and anarchists.

On Friday evening, 17 people were arrested and four police officers were injured in the first clashes in the village of Urrugne near a camp of anti-G7 activists, local authorities said.