European Union chief Ursula von der Leyen said the bloc will respond to any tariffs the US might impose on it, she said in a statement on Tuesday.
"I deeply regret the US decision to impose tariffs on European steel and aluminum exports," von der Leyen said.
"Unjustified tariffs on the EU will not go unanswered — they will trigger firm and proportionate countermeasures," she wrote, adding that the "EU will act to safeguard its economic interests. We will protect our workers, businesses and consumers."
The EU response comes after US President Donald Trump ordered a 25% tariff on foreign steel and aluminum imports that is set to take effect in March.
The tariffs will also apply to imports from Canada and Mexico, countries that are top suppliers of metals to the US.
The Trump administration is hoping the levies will give local producers relief from intense global competition, allowing them to charge higher prices.
The European Commission, the executive arm of the EU, said in a statement that the imposition of tariffs would be "unlawful and economically counterproductive" given the deeply integrated production chains between the EU and US.
"By imposing tariffs, the US would be taxing its own citizens, raising costs for business, and fuelling inflation," the statement said.
