European Commission President Jean Claude Juncker said Trump's tariffs were "totally unacceptable" and promised to retaliate in due course.
"Unilateral responses and threats over trade war will solve nothing of the serious imbalances in world trade".
European officials, bracing for the tariffs, have threatened to retaliate against US orange juice, peanut butter, kitchenware, clothing and footwear, washing machines, textiles, whiskey, motorcycles, boats and batteries.
She calls the USA measures illegal and counterproductive.
President Trump has imposed a 25 per cent tariff on steel imports from the European Union, Canada and Mexico, in a move that the President of the European Commission claimed could push Brussels closer to Moscow.
During a conference call early Thursday, Ross shrugged off questions about the US facing possible retaliation, or whether the move would negatively affect the G7 meeting. "We are awaiting their reaction".
Tariffs might protect some jobs in the USA steel industry, but far fewer than the number of jobs that will be lost. "This is a very strong Canadian action in response to a very bad USA decision".
Canadian Foreign Minister Chrystia Freeland had traveled to Washington on Tuesday to discuss the issue, among other matters, with U.S. Trade Representative Robert Lighthizer.
The EU had earlier agreed to discussions on increased market access for USA products but only if the US first agreed to a permanent exemption from the steel tariffs.
Mr Trump imposed the tariffs in March to address global oversupply of the metals.
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Canada and Mexico were temporarily exempted from blanket tariffs of 25 per cent on steel and 10 per cent on aluminum, with the Trump administration linking a permanent reprieve to a NAFTA deal.
Ross himself heads to Beijing on Friday where he will attempt to get firm deals to export more USA goods in a bid to cut America's $375 billion trade deficit with China.
President Donald Trump's administration has threatened to impose tariffs on auto imports, is engaged in negotiations with China to reduce America's yawning trade deficit and has said it will punish Beijing for stealing its technology by imposing tariffs on $50 billion of imports from China.
Ross met Thursday morning in Paris with French Finance Minister Bruno Le Maire, whose government has vigorously objected to tariffs. News reports say that Trump is determined to keep Germany luxury cars out of the U.S.
The tariffs will also cause major economic disruptions in the US, Canada, Mexico, and the EU.
The Trump administration is making good on its threat to slap Canada with hefty steel and aluminum tariffs, setting the stage for a possible trade war and leaving Prime Minister Justin Trudeau in the middle of a very divided group of leaders at next week's G7 summit.
"We are deeply disappointed that the United States has chose to apply tariffs to steel and aluminium imports from the European Union on national security grounds".
"This is dumb. Europe, Canada, and Mexico are not China, and you don't treat allies the same way you treat opponents", said Sen.
Getting rid of the exemptions for Canada and Mexico could also complicate ongoing negotiations on NAFTA.
"We believe that this combined package achieves the original objectives we set out, which was to constrict imports to a level to allow those industries that operate domestically to do so on a self-sustaining basis going forward".