President Donald Trump issued a threat to impose a 20% tariff on imported cars from the European Union.

The statement, made via Twitter, was made in response to the EU imposing $3.3 billion in tariffs on the United States, after Trump proposed cutting imports on aluminum and steel. The threat could lead to escalating global trade tensions between the two governing bodies. According to a European Commission memo, the EU planned to respond with equivalent tariffs on the U.S.

Following the tweet, Volkswagen AG, Daimler AG, and BMW AG shares dropped in value, while U.S. car companies also saw declines in the stock market.

EU car exports to the U.S. total about $44 billion per year in trade.

Related: Trump Floats Tariffs on Imported Vehicles, Parts