Top European officials held last-ditch talks in Paris with American trade officials to try to avert USA tariffs on steel and aluminum.
But the U.S.is pressing ahead nonetheless, a long-threatened tactic sure to cast a pall over the G7 summit Prime Minister Justin Trudeau is hosting next week in Quebec.
The issue was a key topic of discussion at the meeting of Group of Seven finance ministers and central bankers in the Canadian ski resort of Whistler.
Trump has also asked the U.S. Commerce Department to look into imposing tariffs on imported cars, trucks and auto parts, arguing that they somehow pose a threat to U.S. national security.
The U.S. tariffs coincide with - and could complicate - the Trump administration's separate fight over Beijing's strong-arm tactics to overtake U.S. technological supremacy.
In the fiscal year that ended September 30, the USA government collected $34.6 billion in customs duties and fees. "This is not the way we do business, and certainly not between longstanding partners, friends and allies", she said in a statement.
He vowed to challenge the action in the World Trade Organisation and the North American Free Trade Agreement.
"They must open their markets and take down their trade barriers!"
European Union trade commissioner Cecilia Malmstrom said the tariffs are "further weakening the Trans-Atlantic relations".
Officials at the G7 meeting said the tariffs made it more hard for the group to work together to confront China's trade practices, especially when Beijing, like most G7 members, supports the current World Trade Organization-based trade rules and the United States is seeking to go around them.
French President Emmanuel Macron, who has enjoyed a warm relationship with Trump, "deplored" the USA move declaring it "illegal".
On April 30, he extended for 30 days the temporary exemption from the steel and aluminum tariffs for Canada, Mexico and the European Union to allow for further negotiation.
Harley-Davidson's stock dropped about 1 percent on Friday, while shares of steelmakers U.S. Steel and AK Steel both rose 2.2 percent.
Meanwhile, experts both outside and within the United States condemned Trump's decision to impose tariffs - 25 per cent on steel and 10 per cent on aluminum - on Canada, Mexico and the European Union, calling them foolish and ultimately counterproductive.
Mexico announced "equivalent" measures on a wide range of US farm and industrial products, including pork legs, apples, grapes, cheese, steel and other goods. The EU has said it would respond with tariffs on $3.3 billion in American imports as early as June 20.
Chairing the meeting, host nation Canada's Finance Minister Bill Morneau allowed participants to register grievances with US Treasury Secretary Steve Mnuchin one at a time, according to a Canadian source.
USA and European officials held last-ditch talks in Paris on Thursday to try to reach a deal, though hopes are low and fears of a trade war are mounting. The U.S. team also wants to secure greater intellectual property protections and an end to Chinese subsidies that have contributed to overproduction of steel and aluminum.
The EU said the imposition of the tariffs is "inconsistent with the United States' obligations and rights set out in the WTO Agreement".