President Trump on Saturday threatened the European Union with a tax on cars made in Europe, responding to E.U. pushback against his proposed steel and aluminum tariffs -- the latest fiery rhetoric in a brewing trade war.

"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.," Trump said on Twitter.

Trump's battle cry came after his announcement Thursday of a 25 percent tariff on steel imports and 10 percent of aluminum imports roiled global markets and angered world leaders.

Among them was European Commission President Jean-Claude Juncker, who said Friday that the E.U. could impose tariffs on Harley Davidsons, Levi jeans and bourbon.

"I don't like using the word trade war, but I can't see how this isn't part of warlike behavior," Juncker told German media.

E.U. officials had earlier said that a series of "countermeasures" had been readied to "rebalance" European markets should Trump sign the tariffs next week. Such measures are due to be discussed by officials next week.

TRUMP DECLARES 'TRADE WARS ARE GOOD,' DIGS IN AMID TARIFF TURMOIL

But Trump, who on Friday declared that "trade wars are good and easy to win," seemed nonplussed by the reaction, also tweeting on Saturday that leaders "laugh at what fools our leaders have been."

"No more!" he tweeted.

The E.U.'s pushback was just one of a series of negative reactions from global leaders to his tariff proposal -- one that he had teased throughout his campaign.

China, one of Trump’s main targets when he talked about an unfair trade balance, expressed concern but made no immediate threat of countermeasures.

“The basis for the global recovery is still unstable,” Foreign Ministry spokeswoman Hua Chunying said Friday, according to The Washington Post. “All countries should make concerted efforts to cooperate to resolve the relevant issues, instead of taking trade-restrictive measures unilaterally.”

Even close U.S. allies, while not immediately promising countermeasures, reacted negatively to Trump’s announcement.

“The United States has a two billion surplus on steel with us so we regard the imposition of any new tariffs or any tariffs on steel or aluminum between our two countries as absolutely unacceptable,” Canadian Prime Minister Justin Trudeau told reporters Friday.

The United Kingdom, meanwhile, gave a tempered, but chilly reception to the announcement on Thursday.

“We are engaging with the U.S. on what this announcement means in practice,” a spokesman for 10 Downing Street said. “We have been clear that we are particularly concerned by any measures that would impact the UK steel and aluminum industries.”

The U.K. may be hoping for an exemption, particularly as it is engaged in its own standoff with the E.U. over the implementation over its 2016 decision to leave the union.

But a White House official told The Wall Street Journal that such exemptions were unlikely.

“The president made clear these would be across-the-board tariffs with no exclusions,” the official said. “One problem with exclusions is that it’s a slippery slope. Where do you stop?”

French Finance Minister Bruno Le Maire supported the E.U.’s reaction and said “all options are on the table” in terms of a response.

“The United States needs to know that if it goes ahead with these measures, they will meet with a strong, coordinated and united response from the European Union,” Le Maire said according to Reuters.

Fox News' Serafin Gomez and The Associated Press contributed to this report.