Major U.S. trading partners wasted little time Thursday striking back against a White House move to slap steel and aluminum tariffs on Canada, Mexico and the European Union.

"This is protectionism, pure and simple," said the European Commission's president, Jean-Claude Juncker, as European trade officials issued a list of hundreds of U.S. products, from peanut butter to motorcycles, which would now be subject to tariffs for European consumers.

Mexico, the third-largest U.S. trade partner, said it would penalize U.S. imports including pork bellies, apples, grapes, cheeses and flat steel.

Canada, the second-largest trade partner, behind China, responded with "dollar-for-dollar" tariffs against U.S. steel and aluminum exports.