Washington DC - The Trump administration on Sunday warned that Canada would be "foolish" to think it could win a trade war with the US, predicting a "devastating" impact on its northern neighbor.

Negotiations between Washington and Ottawa broke down late Friday, putting into force new 50% US tariffs impacting about $20 billion worth of Canadian goods, or 5.5% of Canadian exports to the US.

Canada, in retaliation, said it would match US tariffs, with new levies notably targeting the US steel and dairy industries to take effect on September 8.

Transportation Secretary Sean Duffy said Canada would come out worse from tussling with President Donald Trump in a trade war.

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"We're great trading partners, right? But Canada gets the benefit of trading with the US way more than the US gets the benefit of trading with Canada," he told the "Fox News Sunday" talk show.

"To think that they're going to go to war with Donald Trump and actually win that war with the US, I think it's foolish on their part."

He predicted that Canadian Prime Minister Mark Carney would return to negotiations "very, very quickly, because it's going to be devastating for his country."

Carney was defiant on Saturday, announcing retaliatory tariffs on the US after walking away from a "bad deal" on trade as the rift between the longtime allies deepened.

"You're at war when you get attacked. We got attacked," Carney said.

Trump hit back at Canada early Sunday, saying, "Canada wants the benefits of being a State, without being one!!!"

"They have also charged our great farmers, for many years, massive amounts of Tariffs. No more!!!" Trump added in a post on Truth Social.