President Donald Trump escalated threats of a trade war on Saturday, saying he would slap a new tax on European cars if the EU retaliated against his proposed steel and aluminum tariffs.

Trump on Thursday called for tariffs of 25% on steel imports and 10% on aluminum products, a move he said would bolster those industries domestically.

The announcement was met with strong criticism by international trading partners who said Trump's plan could spark a trade war. European Union officials said they would retaliate with new tariffs on U.S. goods, including Harley-Davidson motorbikes, bourbon whiskey and Levi's jeans.

Trump fired back in a tweet Saturday.

"If the E.U. wants to further increase their already massive tariffs and barriers on U.S. companies doing business there, we will simply apply a Tax on their Cars which freely pour into the U.S.," he wrote. "They make it impossible for our cars (and more) to sell there."

The U.S. imported more than 1.2 million European cars from brands like BMW and Volkswagen in 2016, the most recent year for which data is available, according to the European Automobile Manufacturers Association.

The United States has an $800 Billion Dollar Yearly Trade Deficit because of our "very stupid" trade deals and policies. Our jobs and wealth are being given to other countries that have taken advantage of us for years. They laugh at what fools our leaders have been. No more! — Donald J. Trump (@realDonaldTrump) March 3, 2018

Trump's surprise vow to impose a new steel tariff rocked global markets. Economists have warned the tariffs could put U.S. jobs and industries at risk.

Trump stoked further fears of international turmoil on Friday by claiming "trade wars are good" and "easy to win."

Related: Three reasons no one wins a trade war

Supporting Trump's move, however, was his commerce secretary, Wilbur Ross. Ross claimed in an interview with CNN that concerns about the tariff's affect on the U.S. economy are "rubbish." He added that if price hikes trickle down to American consumers, they would not be significant.

Trump also fired off a tweet Saturday saying "very stupid" trade deals are holding America back.

"The United States has an $800 Billion Dollar Yearly Trade Deficit because of our 'very stupid' trade deals and policies," he said. "Our jobs and wealth are being given to other countries that have taken advantage of us for years. They laugh at what fools our leaders have been. No more!"

-- CNN's Stephen Collinson contributed to this report.