"We will now trigger a dispute settlement case at the WTO," EU Trade Commissioner Cecilia Malmström said | Patrick Seeger/EPA EU announces retaliation against Trump tariffs US president has imposed tariffs on Mexico, Canada and the EU.

The EU confirmed Thursday that it would retaliate against steel and aluminum tariffs imposed by U.S. President Donald Trump, in what many European officials fear could be the first salvos of a transatlantic trade war.

Over recent days, European leaders had hoped that Washington would exempt the EU from tariffs, or would instead impose a quota system. Trump, however, ultimately decided to roll out the full 25 percent additional tariff on steel, and 10 percent on aluminum as originally threatened.

"We will now trigger a dispute settlement case at the WTO, since these U.S. measures clearly go against agreed international rules. We will also impose rebalancing measures and take any necessary steps to protect the EU market from trade diversion caused by these U.S. restrictions," said European Commissioner for Trade Cecilia Malmström.

The European Union has already notified the World Trade Organization of plans to levy duties on about $7 billion worth of U.S. exports in response, with the aim of collecting $1.6 billion in tariff revenue.