The European Union’s trade chief dismissed U.S. President Donald Trump’s threat to impose tariffs on car imports, and vowed to “stand up to bullies,” adding to signs that a widening trans-Atlantic rift over protectionism could escalate into an all-out trade war.

“In some places, trade has been blamed for the pains of globalization, others use it as a scapegoat and insist that we can hide behind walls and borders,” EU Trade Commissioner Cecilia Malmstrom said at a conference in Brussels on Monday. “And just recently we’ve seen how it is used to threaten and intimidate us. But we are not afraid, we will stand up to the bullies.”

After announcing tariffs on steel and aluminum imports, Trump reiterated over the weekend that he may now place levies on European cars, telling supporters at a rally that the countries of the EU have banded together “to screw the U.S. on trade.” The latest spat comes as disputes over areas ranging from climate change to Middle East policy strain the bonds holding together the world’s closest political and military alliance.

A meeting in Brussels between Malmstrom and her U.S. counterpart Robert Lighthizer on Saturday ended without a breakthrough, as the EU didn’t receive reassurances that it will be exempted from metal tariffs. Malmstrom said on Monday that “she has read on twitter” that Trump may offer clarifications on which countries may be spared.

The bloc says that it’s a close ally of the U.S. and therefore any import levies on national security grounds are unjustified. While no further bilateral meetings are planned at the moment, contacts are ongoing as the EU is racing against time to secure an exemption before the aluminum and steel tariffs are enacted in less than two weeks.

Asked to respond to Trump’s accusations that the EU is imposing barriers to U.S. automakers, Malmstrom said that “it’s hard to argue on Twitter over these issues, but the European Union is a very open market.” The bloc does impose a 10% levy on U.S. car imports, while the U.S. charges a 25% levy on trucks and pick-ups, and up to 40% on some clothes, she said.

Eliminating these tariffs was the subject of the negotiations for a free trade agreement between the EU and the U.S. which stalled under Trump, according to Malmstrom. In any case, “if the issue is a security issue on trade and aluminum, I fail to see how the car issue fits into that picture.”