Donald Trump and Boris Johnson hold a bilateral meeting during the G7 summit in Biarritz, France

U.S. President Donald Trump on Sunday said he was getting along well with his Western allies at a G7 summit in France, dismissing reports of rifts among leaders as they prepared to discuss global trade woes.

The G7 gathering is taking place against a backdrop of growing worries about a global economic downturn and coincides with an era of international disunity across an array of issues that have strained decades-old allegiances.

"Before I arrived in France, the Fake and Disgusting News was saying that relations with the 6 others countries in the G-7 are very tense, and that the two days of meetings will be a disaster," Trump wrote on Twitter shortly before meeting British Prime Minister Boris Johnson.

"Well, we are having very good meetings, the Leaders are getting along very well, and our Country, economically, is doing great - the talk of the world!"

However, major policy differences over trade protectionism, climate change, Iran and France's push for a universal tax on digital technology giants clouded the run-up to the summit in the Atlantic coastal resort of Biarritz.

Trump arrived in France just hours after escalating his trade war with China in a tit-for-tat battle between the world's two largest economies that has spooked financial markets.

"I am very worried about the way it's going, the growth of protectionism, of tariffs that we're seeing," the UK's Johnson said on Saturday.

"Those who support the tariffs are at risk of incurring the blame for the downturn in the global economy, irrespective of whether or not that is true."

Underlining the discord, Trump also threatened his host on the eve of the summit, saying the United States would tax French wine "like they've never seen before" unless Paris dropped a digital tax on U.S. technology companies.

Leaping into the fray, European Council President Donald Tusk, who takes part in the G7 discussions, warned the EU would respond "in kind" if Trump acted on his threat.

"This may be the last moment to restore our political community," Tusk told reporters on Saturday, giving a bleak assessment of Western relations.

Looking to broaden the scope of the debate, Macron has invited several African leaders to discuss the problems facing their continent, while leaders from India, Australia, Chile and Spain are due to attend a dinner on Sunday where the focus will be on the environment and other issues.

Brexit squabbles

Trump up-ended last year's G7 summit in Canada, walking out of the meeting early and disassociating himself from the final communique having initially endorsed the document.

The delegations had barely arrived in Biarritz before differences were exposed, with senior U.S. officials accusing the host, French President Emmanuel Macron, of looking "to fracture the G7" by focusing on "niche issues" rather than major global concerns.

France denied this, pointing to Sunday's initial session covering the economy, trade and security - areas that used to draw easy consensus but are now sources of great friction.

Overnight, Trump wrote on Twitter that lunch with Macron was the best meeting the pair had yet had, and that a meeting with world leaders on Saturday evening also "went very well."

France has got around the communique problem by doing away with the time-honoured document, deciding it was not worth even trying to find common language.

While the transatlantic rift is the most stark, there are also deep divisions within the European camp, with Johnson making his G7 debut at a time when he is struggling to persuade EU capitals to renegotiate Britain's exit from the bloc.

Johnson and Tusk, who are due to talk on Sunday, sparred ahead of the meeting over who would take the blame if Britain leaves the EU on Oct. 31 without a Brexit agreement acceptable to both sides.

"I still hope that PM Johnson will not like to go down in history as Mr No Deal," said Tusk, who leads the political direction of the 28-nation EU.

Johnson later retorted that it would be Tusk himself who would carry that mantle if Britain could not secure a new withdrawal agreement.

Macron added to the internal EU strains by unexpectedly threatening on Friday to block an EU trade deal with a group of South American states over Brazil's handling of fires that are ravaging the Amazon rainforest.

Germany and Britain both voiced deep concern about the fires, but disagreed with Macron on how to respond, saying shooting down the ambitious Mercosur trade accord would not help save the Amazon.