The European Commission said it still hopes that U.S. President Donald Trump won’t follow through with his plan for imposing punitive import tariffs, as the spat over trade protectionism adds further strain to the trans-Atlantic alliance.

“We hope that eventually this initiative from the U.S. side will not be followed through,” European Commission Vice President Valdis Dombrovskis said in a Bloomberg Television interview ahead of a meeting of the European Union’s executive arm to discuss a response to the U.S. plans. “But it’s also clear that the EU is going to react if these one-sided tariffs are going to be imposed by the U.S.”

The Commission says Trump’s tariffs would upend the rules which have underpinned global commerce since the end of World War II. The latest clash follows a series of of disputes over topics, including action against climate change and Middle East policy, that have tested the ties between the major western powers.

Trump has signaled he may escalate further by imposing tariffs on imports of European cars, while accusing the EU—the world’s largest trading bloc—of making it “impossible” for American auto manufacturers to sell their products to the continent. A meeting with Sweden’s Prime Minister Stefan Lofven on Tuesday failed to ease the tension.

‘Firm, Proportionate’

The EU says its retaliation to Trump’s tariffs will be in line with World Trade Organization regulations. The bloc is preparing to unveil a plan for punitive retaliatory levies later on Wednesday, while insisting that it remains a champion of rules-based free trade.

The plans include a 25 percent tit-for-tat levy on dozens of U.S. products, ranging from steel ladders and frying pans to motorcycles, peanut butter and manicure solutions. Targeting 2.8 billion euros ($3.5 billion) of iconic American goods produced in key Republican constituencies, the EU hopes to raise pressure on Trump to ditch his steel and aluminum tariff plans.

“We are assessing what are the options of our action,” Dombrovskis said. “We will react in a firm and proportionate way within WTO rules.”