Canadian Prime Minister Mark Carney has pledged to match US tariffs "dollar for dollar" after trade talks between Ottawa and Washington broke down.

Hefty new 50% duties impacting around $20 billion worth of Canadian goods took effect on Saturday after Carney said "last-minute changes in the U.S. proposed terms were unfair, uneconomic, and called into question the reliability of any deal."

"Canada will match those tariffs dollar for dollar to protect our workers and businesses," Carney added in a statement. "In recent weeks, we made important progress toward improving Canada’s position as having the best deal in the world with the U.S. However, that progress has not been enough to meet our objectives for Canadians."

US Trade Representative Jamieson Greer said on Friday that "tonight, Canada declined to finalise the trade deal under the terms agreed earlier this week." He added that the Trump administration had tabled "significant tariff reductions on steel, aluminium, autos and lumber" in exchange for Canadian concessions.

The announcement came despite US President Donald Trump having said the US "should be able to have a deal with Canada" as he touted his "good relationship" with Carney.