BRUSSELS—The European Union fired its first shot Friday against U.S. steel and aluminum tariffs, launching a World Trade Organization challenge and vowing swift duties on American exports, in a sign that the bloc would go blow-for-blow with President Donald Trump over trade.
Brussels says Washington’s measures are “pure” protectionism. With its WTO complaint, it is challenging the U.S. national-security justification for Mr. Trump’s levies, in an unusual rift between the trans-Atlantic allies.
“The U.S. is playing a dangerous game here,” European Trade Commissioner Cecilia Malmström said Friday. For Europe, “Not responding will be the same as accepting these tariffs, which we consider illegal under WTO rules.”
An EU-U.S. trade war risks undermining joint efforts to counter a common problem, China.
Highlighting Europe’s desire to compartmentalize differences with the U.S. and engage Washington in challenging Beijing’s state-backed capitalism, the EU on Friday also filed a WTO case against China for undermining intellectual-property rights with forced technology transfers.
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Even as Brussels signals its willingness to cooperate with the U.S. to uphold the global-trade order that the Western allies built after World War II, the path forward in mending bilateral relations remains unclear.
“We are not going to enter into any negotiations,” Ms. Malmström said, rebuffing U.S. Commerce Secretary Wilbur Ross’s suggestion Thursday that Washington and Brussels could hammer out a trade deal despite the tariffs.
The European Commission—the EU’s executive arm—has prepared a list of U.S. exports that could be slapped with €6.4 billion ($7.5 billion) worth of levies, €2.8 billion of which could go into effect June 20.
EU leaders will discuss their course of action one final time with the commission before Brussels pulls the trigger, according to European officials familiar with the deliberations.
Meanwhile, an investigation into vehicle and auto-part imports Mr. Trump ordered last month is stoking uncertainties and threatening to escalate the EU-U.S. trade spat.
The president had repeatedly warned that if the EU counters his steel and aluminum tariffs, he would hit back with levies on European car exports. His latest probe, also on national security grounds, could open a path for retaliation.
“We will see where that goes,” Ms. Malmström said. “Should that happen, that is extremely worrying.”
European leaders have rejected White House suggestions that EU countries—22 of which are North Atlantic Treaty Organization members—pose security threats to the U.S.
Mr. Trump’s “America First” policies are undermining efforts to combat causes of global trade imbalances, driven by Chinese overcapacity, subsidies, and Beijing’s other trade practices, EU officials say. They also threaten to torpedo the international system that, despite flaws, broadly protects everyone’s interests, EU officials say.
“We are determined to protect the multilateral system,” Ms. Malmström said. “Protectionism can never be a solution.”
Write to Emre Peker at emre.peker@wsj.com