A truck crosses into the United States on the Gordie Howe International Bridge, connecting Windsor, Ontario, and Detroit, Michigan, on September 6, 2025. [AFP]

Canada's retaliatory tariffs on billions of dollars in US products took effect Tuesday, as a trade war between the North American neighbors heats up.

The announced duties of 15 percent, 25 percent and 50 percent apply to $27.6 billion (US$20 billion) in imports from the United States, covering steel and aluminum products as well as dairy goods like cheese. But Canada removed some seafood products from the initial list.

Ottawa's pushback comes weeks after US President Donald Trump imposed 50-percent tariffs on a similar value of Canadian products, over what Washington deemed as "discriminatory treatment" against US alcohol, automobile and dairy industries.

The US tariffs hit items like hockey sticks and cement, impacting about 5.5 percent of Canadian exports to the United States.

On Monday, Trump threatened to block sales of Canada's Bombardier Aviation in the United States, unless the Quebec-based plane maker moves manufacturing to the US.

"No more selling Bombardier in the United States!" Trump posted in all caps on his Truth Social platform, though he did not specify how he would achieve a sales halt.

Thousands of Bombardier aircraft currently operate in US airlines' domestic fleets.

In a statement Monday, the aerospace company touted its creation of "tens of thousands of jobs across the United States," with "direct employment" in more than 20 states, including Kansas, Texas, Arizona and California.

The company also noted that it spends over $2.5 billion annually with suppliers, and said its supply chain is "made up of approximately 2,800 American companies across 47 states."

"Bombardier values its great partnership with American companies and its US employees," the company's statement said.

US tariffs pose a modestly negative risk to Canada's overall economy, but analysts note that they have a sharper impact on Central Canada's manufacturing sector.

Negotiations between both sides broke down August 21 after days of meetings in Washington, with Canadian Prime Minister Mark Carney saying he decided to suspend the trade talks.

At the time, Carney said the Trump administration's terms were ultimately unacceptable, adding that US negotiators had introduced restrictions on Canadian trade deals with other countries at the eleventh hour.

US officials also made unacceptable "threats" to the French language and "Quebec culture," he added, referencing the French-speaking province in eastern Canada.

But Trump's top trade official Jamieson Greer later noted that the US government is aware that French language protections are sensitive and important.

"This is not something where we push hard, or condition, or red-line," he told Canadian public broadcaster CBC last month.

Ottawa and Washington have not resumed negotiations since, and have continued trading barbs.

Asked if both sides were in a trade war, US Treasury Secretary Scott Bessent told CNBC last week (Aug 31) that he did not think one could be in a tit-for-tat fight with a country that is 13 times larger.

"We're not at war with Canada," Bessent said on the sidelines of a G20 finance leaders' meeting. "How are we going to be at war with Canada? They're going to take their two set submarines from the Edmonton Mall and sic them on us?"

He was referring to a former attraction at a shopping center in the Alberta province.

Pentagon chief Pete Hegseth separately appeared to mock the physical appearance of a Canadian soldier on social media.

Carney said a day later that the remarks were "beneath their office," adding that this was "not constructive."

He said both sides can have discussions "when the Americans stop doing memes" and start being serious.

Trump also signed an order in August to rename Lake Ontario, on the border with Canada, as "Lake America."

This has sparked anger in Canada, amid a broader wave of patriotism triggered by Trump's hostility.

At the start of his second presidency, Trump also ordered the Gulf of Mexico be renamed the Gulf of America. He has made claims that Canada should become the 51st US state as well.

Although Carney is backed by Canadian public opinion, his country remains reliant on its neighbor. Nearly 60 percent of Canada's imports come from the United States and about 70 percent of its exports go to the US market.

To help businesses and workers, Canada's government has unveiled an aid package of $7.5 billion (US$5.4 billion).