The relationship between the US and its allies was already off-balance after Mr. Trump just a week before the summit imposed 25% tariffs on steel and 10% tariffs on aluminum from the European Union and Canada.
Navarro's initial remarks came during an interview on Fox News Sunday after the G-7 economic summit in Quebec.
The escalating clash cast a shadow over the Trump's nuclear talks with Kim and led critics to question why the president was bashing USA partners while appearing to cosy up to one of Washington's bitterest long-time foes.
Trudeau also said he would stand up for Canadians.
"That's unconscionable", Heyman said."Anybody who represents the United States of America from the White House using that kind of language with any world leader of any type, I think is uncalled for".
Trump had informed the Western allies of his plans to impose new tariffs on steel and aluminum imports from Canada, the European Union and Mexico. "That's what weak, dishonest Justin Trudeau did", said Navarro, referring to the joint press conference that is routinely held at the end of the annual events.
"That's going to cost a lot of money for the people of Canada", the irate president said. Watching the US president use social media to draw attention to himself and diminish his adversaries, real or imagined, is a daily feature of his presidency. "Our Tariffs are in response to his of 270% on dairy!" Trump's chief economic adviser Larry Kudlow accused Trudeau of "stabbing the United States in the back" and said Trump's Canadian counterpart had attempted to undermine the president on the eve of an important summit with North Korea.
Bruce Heyman, a former USA ambassador to Canada, said Mr Navarro had "crossed a line".
"If the USA stopped joint military exercises with the South Koreans, it could help moderate Kim Jong Un's behavior", the report continued.
Navarro, a supporter of tariffs to help reduce the U.S.'s trade deficit and a longtime critic of China, turned his anger at Canada over the weekend as a Group of Seven meeting hosted by Trudeau ended in disarray and trade threats. That would devastate the Canadian vehicle industry, which counts on the USA for 85 percent of its sales, according to industry groups.
It's not clear what new costs to Canada Trump was referring to in Singapore. The U.S. had a trade surplus with Canada of $8.4 billion a year ago, according to the Office of the United States Trade Representative.
The northern nation is trying to pivot away from the US, but it's not easy. The prime minister is also nationalizing a pipeline to reduce reliance on American buyers.
On Sunday, Trump's aides took to cable news to blast the PM further.
He then asserted multiple times to the room full of reporters that he has a "good relationship" with Trudeau. Americans are thankful for Canadian artists like Leonard Cohen, Celine Dion and Drake as well as more significant historical acts like when the people of Gander, Newfoundland took in those whose planes were grounded in the town following the 9/11 attacks.
Trudeau, meanwhile, can look back to his father's legacy for support.
But Trudeau - like many prime ministers before him - has little room for maneuver.
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