U.S. President Donald Trump threatened a 20 percent tariff on cars imported from the European Union unless the bloc removes import duties and other barriers to U.S. goods, escalating a global trade war the EU warned could endanger $300 billion in commerce.

"Based on the Tariffs and Trade Barriers long placed on the U.S. and its great companies and workers by the European Union, if these Tariffs and Barriers are not soon broken down and removed, we will be placing a 20% Tariff on all of their cars coming into the U.S. Build them here!" Trump said in a tweet on Friday.

The EU planned to retaliate, according to a European Commission memo obtained by Bloomberg. "An introduction of U.S. tariffs would be met with equivalent penalties imposed by affected trading partners," it said.