Le Maire said US President Donald Trump's plans to impose tariffs on steel and aluminium imports would, if confirmed, be "unacceptable".
He called for a "strong, coordinated and united response from the EU".
Trump triggered a furore on Thursday by announcing he would slap tariffs of 25 percent on steel and 10 percent on aluminium to protect US producers.
The announcement caused an outcry among US allies such as Canada, the EU, Mexico and Australia as well as China, the world's biggest steel producer.
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A defiant Trump reacted by saying that a trade war would be "good" for the US economy.
"When a country (US) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win," Trump tweeted.
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