The second issue was United States' insistence on "reciprocal" trade, the source said.
Mocked for cultural gaffes on a recent trip to India, and slammed by others for his efforts in renegotiating the North America Free Trade Agreement at Trump's behest, Trudeau has also been attacked by the left-wing for his decision to nationalize an oil pipeline that had bogged down in protest and controversy. The prime minister's office has stood by Trudeau, releasing a statement saying he has been honest with Trump throughout the summit. "I don't understand the obsession with trade relations with Canada", he said on Fox's "Sunday Morning Futures", given that Canada is the biggest single buyer of American goods and services in the world.
Trump is in Singapore where he is meeting with North Korean leader Kim Jong-Un. The Trudeau government has announced it will impose dollar-for-dollar, retaliatory tariffs on metals and a range of other US products by July 1.
Listen above to hear what kind of impact these retaliatory tariffs might have on the US, why George says the tariffs will "hurt both sides", and how the global bridge between MI and Canada fits into the countries' trad relationship.
The venom of the exchange sent shockwaves through foreign policy circles and rattled markets, as another sign of the weakness of the Western alliance and of the threat NAFTA will collapse.
Canada's foreign minister, Chrystia Freeland, said her country "does not conduct its diplomacy through ad hominem attacks".
But even conservative and populist figures on the right of Canadian politics, some of them admirers of Trump's brand of politics when it stays south of the border, rallied to their liberal premier.
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Trump said Trudeau's promise not to back down from U.S. tariffs is "going to cost a lot of money for the people of Canada".
On Monday, MPs in the House of Commons approved a motion denouncing Trump's name-calling tirade and endorsing Trudeau's decision to stand his ground against US tariffs and tweeted presidential threats. "$800 Billion Trade Deficit ..."
Behind his claim lies a difference in how the countries' calculate the trade figures.
Trump's tough rhetoric for the leader of Canada, one of the United States' closest allies, was in sharp contrast with the words he chose to describe Kim.
If these goods that originate in Canada and are eventually sold there without have been substantially worked upon in the United States are taken into account then Ottawa comes out the victor.
In reality, the United States has a small surplus with Canada when goods and services are considered - $341.2 billion to Canada's $325.9 billion, according to 2016 statistics from the USA trade representative.
On Saturday, the summit's second and final day, President Trump left early and took to Twitter to praise Canada, seemingly leaving the country on positive terms. In the course of discussion, a compromise was reached, with the final wording speaking about commitment to "modernize the WTO to make it more fair as soon as possible". "Canadians, we're polite, we're reasonable, but we also will not be pushed around".
Meanwhile, the Canadian province of Quebec has offered $C100 million ($A101 million) in loans and guarantees for steel and aluminium companies hit by the U.S. tariffs. Nor did the U.S. administration want to sign a charter regarding plastic in the ocean.