There is a "special place in hell" for Canadian Prime Minister Justin Trudeau, a top US official has said.
Canada and the US have exchanged tough words following the G7 summit, though the Canadian response has been somewhat more measured in its tone.
It began after Mr Trudeau told reporters Canada "will not be pushed around" by US President Donald Trump, who is raising tariffs on imports from its northern ally.
Mr Trump responded by calling Mr Trudeau "meek and mild" and "dishonest and weak". He then said the US would not sign a communique prepared during the G7 that promised the nations would "achieve a clean environment, clean air, and clean water" and create "a healthy, prosperous, sustainable and fair future for all".
The US upped the ante when Larry Kudlow, director of the National Economic Council, said Mr Trudeau "kind of stabbed us in the back".
Mr Kudlow said it was important that Mr Trump looked strong ahead of his unprecedented meeting with North Korean dictator Kim Jong-un on Tuesday.
Canada in response said Mr Trudeau hadn't said anything he hadn't said before - that Canada would have to reluctantly retaliate with its own tariffs.
"Canada does not conduct its diplomacy through ad hominem attacks," said Canadian Foreign Minister Chrystia Freeland, "and we refrain particularly from ad hominem attacks when it comes from a close ally."
But on Sunday evening (local time) the White House threw decorum to the wind. US trade adviser Peter Navarro told Fox News Sunday there is "a special place in hell for any leader that engages in bad faith diplomacy with President Donald J Trump and then tries to stab him in the back on the way out the door, and that's what bad-faith Justin Trudeau did with that stunt press conference, that's what weak dishonest Justin Trudeau did".
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Mr Trudeau declined to respond to the extraordinary attack, instead directing questions to Ms Freeland. She said she has an upcoming meeting with US trade representatives, where she hoped the North American Free Trade Agreement would be renewed.
"We are convinced that common sense will triumph," she said.
Canadian newspaper Toronto Star described Mr Navarro's outburst as a "baffling and unprecedented attack" and "a level of public vitriol not seen in Canada-US relations in more than 50 years".
The attack from the south has united Canadian politics, with the Canadian leader of the opposition saying they support Mr Trudeau's efforts "to make the case for free trade".
"Divisive rhetoric and personal attacks from the US administration are clearly unhelpful," tweeted Andrwe Scheer.
Former Canadian Prime Minister Stephen Harper said Mr Trump was targeting the wrong nation.
"From what I understand of American public opinion, I don't think even Trump supporters think the Canadian trade relationship is a problem," he told Fox News.
Mr Trump at the weekend threatened to end all trade with US allies if they didn't drop their "ridiculous and unfair" tariffs.