European and world leaders have promised swift retaliation after Donald Trump signed proclamations imposing 25% tariffs on all steel and aluminium imports to the US in a move that risks precipitating a global trade war.

The president of the European Commission, Ursula von der Leyen, said on Tuesday she “deeply regretted” the US president’s decision, announced late on Monday, adding: “Unjustified tariffs on the EU will not go unanswered.”

Von der Leyen said the US tariffs would “trigger firm and proportionate countermeasures” and vowed the 27-nation bloc would “act to safeguard its economic interests. We will protect our workers, businesses and consumers.”

The EU’s trade commissioner, Maroš Šefčovič, told the European parliament that Trump’s move would fuel inflation but that the bloc remained committed to finding a mutually beneficial solution as soon as possible.

“The EU sees no justification for the imposition of tariffs on our exports, which are economically counterproductive,” he said. “Tariffs are taxes, bad for businesses, worse for consumers … and harmful to the global trading system.”

Šefčovič said the commission, the EU’s executive body, was “assessing the scope of the measures announced overnight and will be responding in a firm and proportionate way by countermeasures”.

Trump’s move raises the US tariff rate on aluminium from 10% to 25%, and eliminates all country exceptions, quota deals and product-specific tariff exclusions for both metals with effect from 4 March.

He said it would simplify tariffs on the metals “so everyone can understand … it’s 25%” on all countries. He has also promised reciprocal tariffs on all countries that tax imports from the US and said he is looking at tariffs on cars and pharmaceuticals.

Trump imposed tariffs on steel and aluminium from dozens of countries in his first term, drawing EU countermeasures on emblematic US goods including Harley-Davidson motorcycles, bourbon whiskey, jeans and orange juice.

According to US trade figures, Canada, Brazil and Mexico are the three biggest steel exporters to the US, followed by South Korea. An estimated 25% of EU steel exports go to the US, worth about €3bn a year over the past decade.

Canada’s prime minister, Justin Trudeau, who is attending an artificial intelligence summit in Paris, said on Tuesday that Canadians would “stand up strongly and firmly if we need to”, describing the Trump administration’s move as “unacceptable”.

South Korea’s acting president, Choi Sang-mok, said Seoul was “committed to protecting the interests of our companies and reducing uncertainties” and would seek to “build a close relationship” with Washington and explore diplomatic options.

Germany’s chancellor, Olaf Scholz, said the EU would present a united front. “If the US leaves us no other choice, the EU will respond as one,” he told parliament. “As the world’s largest market, with 450 million citizens, we have the strength to do so.”

France’s industry minister, Marc Ferracci, said that while his country was not a major producer of aluminium or steel, it could still be indirectly affected if Chinese exports discouraged by the US tariffs instead made their way to European shores.

“We should expect that there will be some effects from Donald Trump’s tariffs,” Ferracci said. “Europe must respond in a united manner, because we know that his strategy is to divide Europeans. We must respond in a firm manner.”