US President Donald Trump threatened his French hosts with taxes on their wines Friday as he headed to a G7 summit in France with a series of parting shots, including another broadside at the Federal Reserve's chairman.

French policemen stand guard near the Grande Plage beach ahead of the G7 summit in Biarritz, France, August 22, 2019. (Photo: VCG)

Trump blew up last year's G7 summit in Quebec, plunging into a bitter row with Canadian Prime Minister Justin Trudeau and refusing to sign the group's traditional joint declaration.

And on issues such as trade tariffs, global warming, Iran, Brexit as well as how to handle Brazil's right-wing President Jair Bolsonaro, Trump's America now stands in opposition to several of its closest allies.

"I think it will be very productive," Trump told reporters outside the White House shortly before beginning his trip to France where he will meet with the other leaders of the world's seven wealthiest countries from Saturday.

G7s always used to be cozy affairs, giving the leaders of like-minded, capitalist democracies a chance to relax and talk frankly in the context of largely shared values.