The other leaders of G7 nations must not be afraid to reach agreements without US President Donald Trump at their upcoming summit, French President Emmanuel Macron said Thursday, as a trade war between Washington and its allies looms.

Macron and Canadian Prime Minister Justin Trudeau held a joint press conference after private talks the night before to form a united front ahead of the Group of Seven summit that begins Friday.

While both Macron and Trudeau have forged cordial relationships with Trump, they made it clear they would not be intimidated by the US leader -- especially in the face of US tariffs on steel and aluminium imports from Canada, Europe, Japan and Mexico.

"The G7 is an opportunity for us to get together and have frank, open discussions among nations who have long been allies and friends," Macron told reporters, with Trudeau at his side.

"There will be subjects on which the (US) president is not totally in sync with the others -- I'm thinking of course of climate change and trade," the French leader said.

"Our common goal is to try to find a text that can be signed by everyone. (...) That said, I agree with what Justin Trudeau has said many times -- the desire to sign a common G7 declaration must not outweigh the need to be mindful of the content," he explained.

"We Europeans and the Japanese are not ready to give up everything to get" Trump's signature, he said, adding that would be a "mistake."

Trudeau heaped scorn on Donald Trump's national security justification for the new metal tariffs. "It is laughable to say that Canada, France... can represent a threat to America's national security, as we are the best allies that the US has had for a long time," Trudeau said. He and Macron also noted that the tariffs would hurt American workers.

The leaders will join Trump, German Chancellor Angela Merkel, British Prime Minister Theresa May, Japanese Prime Minister Shinzo Abe and new Italian Prime Minister Giuseppe Conte in Quebec for talks on Friday and Saturday.