The European Union vowed to respond to 25 percent tariffs President Donald Trump said the US will impose on steel and aluminum imports, escalating a potential trade dispute with one of Washington’s closest allies.

“I deeply regret the US decision to impose tariffs on European steel and aluminum exports,” European Commission President Ursula von der Leyen said in a Tuesday statement. “Unjustified tariffs on the EU will not go unanswered — they will trigger firm and proportionate countermeasures.”

The tariffs follow through on Trump’s complaint that the EU treats the US “very unfairly,” and he often cites the bloc’s trade surplus with the US as a reason for punitive measures. The new rates will take effect on March 12, according to a pair of proclamations issued by the White House late Monday.

The EU has prepared multiple lists of American goods to hit with retaliatory tariffs if Trump moves forward with levies, modeling various possibilities depending on what the initial US salvo looks like, Bloomberg reported earlier.