• U.S. President Donald Trump has threatened to impose "far larger" tariffs on the European Union and Canada if they work together to combat trade tariffs.
• "If the European Union works with Canada in order to do economic harm to the USA, 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!," Trump posted on social media platform Truth Social on Thursday.

U.S. President Donald Trump threatened to impose "far larger" tariffs on the European Union (EU) and Canada if they work together to combat trade tariffs.

"If the European Union works with Canada in order to do economic harm to the USA, 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!," Trump said in a Truth Social update on Thursday.

On Wednesday, the White House leader had announced that he will set a 25% tariff on "all cars that are not made in the United States" with the levies due to take effect on April 2.

Trump White House aide Will Scharf said the new duties apply to "foreign-made cars and light trucks" and come in addition to tariffs that are already in place. He estimated that the measures will result in "over $100 billion of new annual revenue" to the U.S.

Global markets were roiled as a new front opened in the burgeoning global trade war, with shares of U.S. and Asian automakers falling after the announcement, and European auto giants expected to do the same at the market open on Thursday.

Trump has already upended long-established global trading relationships by imposing import duties on goods coming from Mexico, Canada and China, as well as levies on all steel and aluminum imports, because of what he sees as unfair trade deficits that the U.S. has with a number of its largest trading partners.

The EU and Canada have yet to issue any statement suggesting they would unite to harm the U.S., as Trump's post indicated, but both have signaled that they could retaliate to the latest impositions.

European Commission President Ursula von der Leyen said the EU "will continue to seek negotiated solutions, while safeguarding its economic interests."

"Tariffs are taxes — bad for businesses, worse for consumers equally in the U.S. and the European Union," she said in a statement.

Canada's new Prime Minister Mark Carney meanwhile described Trump's move as "a direct attack" and told reporters he would be convening a high-level cabinet meeting on Thursday to decide on a response.

"We will defend our workers, we will defend our companies, we will defend our country, and we will defend it together," he said in Kitchener, Ontario, according to Reuters.

— CNBC's Kevin Breuninger and Eamon Javers contributed reporting to this story.