Already embroiled in a high-risk trade war with China, Trump warned late Friday he would impose punishing tariffs on French wine if France doesn't withdraw a new digital tax that will hit US tech giants.

The European Union "will respond in kind," EU Council President Donald Tusk answered to Trump, who issued his threat on departing Washington for Biarritz.

Host Emmanuel Macron, France's president, and even British Prime Minister Boris Johnson also sounded the alarm about the dangers of Trump's escalating trade war with China.

"I am very concerned. The UK is at risk of being implicated in this. This is not the way to proceed," Johnson told reporters on the plane to the G7 summit in Biarritz.

"I want to see a dialling down of tensions."

The stormy start to a summit of what are meant to be like-minded allies -- Britain, Canada, France, Germany, Italy, Japan and the United States -- led Tusk to sound a dire alarm over the future of Western leadership.

"This may be the last moment to restore our political community," he said. "The world needs our cooperation."

In an attempt to lighten the mood, Macron deployed his secret weapon of French cuisine diplomacy, treating Trump to a surprise lunch minutes after he had arrived on Air Force One.

Speaking to reporters in fluent English, Macron called Trump "a very special guest".