European Commission President Ursula von der Leyen on Tuesday vowed "firm and proportionate countermeasures" in response to President Donald Trump's decision to impose 25% tariffs on steel and aluminum imports.

The Details: "Unjustified tariffs on the E.U. will not go unanswered — they will trigger firm and proportionate countermeasures," Von der Leyen said in a statement.

Von der Leyen did not detail which countermeasures are being considered, though it is expected that the E.U. will move to protect its domestic steel and aluminum industries.

Maroš Šefčovič, European Commissioner on trade and economic security, called Trump's tariffs "a lose-lose scenario" and that the "time has come" for Europe to respond.  

"We are currently assessing the scope of the measures announced overnight and will be responding in a firm and proportionate way by counter measures," he told members of the European Parliament on Tuesday.

According to the U.S. Commerce Department, the U.S. imported approximately four million metric tons of steel and 283,000 metric tons of aluminum from Europe in 2022.

Stock To Watch: Shares of steel companies, including United States Steel Corp. (NYSE:X), Steel Dynamics, Inc. (NYSE:STLD) and Nucor Corp. (NYSE:NUE), have reacted positively to Trump's tariffs set to take effect in March.

Investors can also monitor the steel industry through the VanEck Steel ETF (NYSE:SLX) which provides targeted exposure to the steel sector.

Shares of aluminum companies like Alcoa Corp. (NYSE:AA) and Century Aluminum Co. (NYSE:CENX)  are gaining Tuesday as investors digest the news.

Century Aluminum released a statement on Monday in support of the new tariff structure.

"Today's executive order implements a 25% tariff on all primary aluminum imports into the U.S. from all foreign countries and closes a loophole in the existing tariff that allowed nearly 75% of imports to avoid the tariff," the company said.

"Closing these loopholes will protect good American jobs and our national security."