"The fact is, Canada is not making concessions in areas where we think they're essential", US Trade Representative Robert Lighthizer said in NY on Tuesday, as quoted by The Hill.
"That's the mother lode", he said.
"That's the motherload, that's the big one", Trump told reporters.
The comments also mark a new low in relations between the two leaders.
United States producers actually sell more dairy product to Canada than they import, and the 300 percent tariff only applies to goods above the quota, which the USA does not meet, according to trade experts.
Trump has demanded major changes to NAFTA, which he has said caused USA manufacturing jobs to move to low-wage Mexico.
A spokesperson for Trudeau, however, said this was not true and that Trudeau never sought a meeting with Trump during the NY gathering.
"We're going to go ahead with Mexico", he said.
As the NAFTA negotiations are wrapping up, and after a successful renegotiation of the US-South Korea free-trade agreement, the Trump administration's focus appears to be shifting to trade talks with Japan.
Canadian officials have said that despite the USA threats to go it alone with Mexico, they do not believe Trump can by himself turn the 1994 pact into a bilateral deal. My job is very simple.
Bank of Canada Governor Stephen Poloz is due to speak on Thursday, while Canadian gross domestic product data for July is due on Friday.More news: Beluga whale makes waves as it swims up Thames near London
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MacNaughton made the comments in response to a question about whether Canada might simply call Trump's bluff and make a take-it-or-leave-it offer, daring Trump to carry out his tariff threat.
At the end of his statement, Trump said that he loves Canada and he has "so many friends" from the country.
Mexico and Canada have historically been united in opposition against any changes in trade policy with the US, but Mexico's leadership has been reluctant to stall any further progress.
Canada has nevertheless rejected the mounting pressure from the U.S., and indicated it was possible the countries may fail to conclude a pact. "Will you be pulling out of NAFTA?" the reporter followed up.
"It's been great for Canada, It's been great for Mexico, but very bad for us".
But Trump didn't seem phased by this. "We are looking for the right deal". "We are going to tax the cars that come in".
He resolved, "We will put billions and billions of dollars into our treasury and frankly, we'll be very happy because it's actually more money than you can make under any circumstance with making a deal".
USA business groups, alarmed by the potential disruption to the three increasingly integrated economies, have lobbied the White House to keep NAFTA as a trilateral deal.
"I don't like NAFTA". "I refuse to use it".
A source told the National Post Wednesday that Jared Kushner had been trying to orchestrate a last-minute summit between Trump, his father-in-law, and Trudeau in the hope of breaking logjams in the trade talks. He said the USA team thinks the Canadian "negotiators have taken advantage of our country for a long time".
- David J. Lynch and Erica Werner contributed to this article.