Washington DC - President Donald Trump threatened to place even larger tariffs on Canada and the European Union (EU) if they work together to develop a response to the US trade war.

"If the European Union works with Canada in order to do economic harm to the USA," Trump said in a post on Truth Social, "large scale Tariffs, far larger than currently planned, will be placed on them both in order to protect the best friend that each of those two countries has ever had!"

The comments follow Trump's decision to slap a 25% tariff on all vehicles that are manufactured outside of the US and then imported into the country.

Trump said that the executive order, which will take effect on April 3, was designed to force manufacturers to make cars "all in one location."

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"You know," he said. "Right now a car would be made here, sent to Canada, sent to Mexico, sent all over the place, it's ridiculous."

As of writing, there has been no public announcement or suggestion that Canada and the EU will work together on countering Trump's vehicle tariffs, though they are thought to be working together to support Ukraine.

The EU and Canada have both responded strongly to the news, with European Commission President Ursula von der Leyen describing it in a statement as "bad for businesses, worse for consumers."

Canadian Prime Minister Mark Carney described the move as a "direct attack" on workers, and vowed to respond strongly.

"We will defend our workers, we will defend our companies, we will defend our country, and we will defend it together," he told reporters in Kitchener, Ontario.

Von der Leyen made a similar commitment to European workers, saying that, "We will jointly protect our workers, businesses, and consumers across our European Union."