The EU has threatened "all necessary measures" in response
The US is to impose tariffs on steel and aluminium imports from the European Union, Mexico and Canada.
The 25% tariffs on steel and 10% on aluminium will begin at midnight.
The move, revealed by Commerce Secretary Wilbur Ross, will affect several key US allies - and Nato members - in Europe.
The UK was "deeply disappointed" by the US decision, while Europe's trade commissioner Cecilia Malmstrm called it a "bad day for world trade".
Mr Ross announced the sanctions from Paris, where he had been negotiating with EU leaders who were attempting to fend off the duties.
He said talks had not made enough progress to warrant further reprieve and acknowledged the possibility of retaliation.
"We will have to see what's their reaction," he said. "We continue to be quite willing and indeed eager to have discussions with all those parties."
French Finance Minister Bruno Le Maire had said the tariffs would be "unjustified and dangerous".
The EU would take "all necessary measures" to respond if the US did impose tariffs, Mr Le Maire warned.
"It's entirely up to US authorities whether they want to enter into a trade conflict with their biggest partner, Europe," he said before the announcement was made.
Ben Digby, of UK business lobby group the CBI, called the tariffs "deeply concerning" but urged the parties to be cautious, given the potential costs of a trade war.
"Now is not the time for any disproportionate escalation, and we urge the EU to consider this when initiating its response," he said.