The North American neighbours swung sharply towards a diplomatic and trade crisis on Sunday as top White House advisers lashed out at Canadian Prime Minister Justin Trudeau with White House trade adviser Peter Navarro telling Fox News Sunday that "there is a special place in hell for any leader that engages in bad faith diplomacy" with Trump.

White House economic adviser Larry Kudlow accused Trudeau of betraying Trump with "polarising" statements on trade policy that risked making the US leader look weak ahead of a historic summit with North Korean leader Kim Jong Un.

"[Trudeau] really kind of stabbed us in the back," Kudlow, the director of the National Economic Council who had accompanied Trump, said on CNN's State of the Union.

But Trudeau got direct personal support from the other G7 leaders with European Council President Donald Tusk tweeting: "There is a special place in heaven for @JustinTrudeau."

British Prime Minister Theresa May "is fully supportive" of Trudeau and his leadership, a senior UK government source said.

Canadian Foreign Minister Chrystia Freeland reiterated her country would retaliate to US tariffs in a measured and reciprocal way.

"Canada does not conduct its diplomacy through ad hominem attacks ... and we refrain particularly from ad hominem attacks when it comes to a close ally," she said.

Freeland, asked about support from allies, said: "The position of our European allies, including Japan, is the same as ours. We coordinated very closely with the European Union, with Mexico, on our list of retaliatory measures and actions."

Germany and France, who sharply criticised Trump's decision to abruptly withdraw his support for a summit communique hammered out at the Canadian summit on Saturday, accused him of destroying trust and acting inconsistently.

Europe would implement counter-measures against US tariffs on steel and aluminum just like Canada, German Chancellor Angela Merkel said.

Trudeau's office said he had not said anything in his closing G7 news conference he has not said to Trump before.

The majority of Canadian exports go to the United States, making Canada uniquely vulnerable to a US trade war.

France was also standing by the G7 communique, a French presidency official said.

"This is of great concern to the G7. I do not know where we go from here. It is impossible to predict what Trump will do next," said a senior G7 official on the summit outcome.

"The world as we know it, namely the US-led rules-based multilateralism, is now in serious danger of unravelling, as illustrated at the G7 meeting," said Erik Nielsen, chief economist at Unicredit Bank.

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South African President Cyril Ramaphosa, who met with Trudeau on Sunday, said it was time for G20 nations to play a role and to "also bring about some good sense to all the key players." "The G20 helpfully has more countries that have a stake in the economy of the world, including the G7," he said.

Canadian officials, including Trudeau, have fanned across America as part of a months-long charm offensive to appeal to pro-trade Republicans at every level. But even those vested in Canadian trade are not expected to come to Trudeau's defence as long as the US economy is roaring.

"I think the pushback by Congress is going to come up incredibly short," Chris Barron, a pro-Trump Republican strategist, said of Republican efforts to rein in Trump.

'Reuters