Canada and Mexico retaliated against the US decision on Thursday to impose tariffs on steel and aluminum imports and the European Union had its own reprisals ready to go, reigniting investor fears of a global trade war.
The tariffs, announced by US commerce secretary Wilbur Ross in a telephone briefing on Thursday, ended months of uncertainty about potential exemptions and suggested a hardening of the Trump administration's approach to trade negotiations.
The measures, touted by president Donald Trump in March, drew condemnation from Republican lawmakers and the country's main business lobbying group and sent a chill through financial markets.
The Dow Jones Industrial Average lost 1 per cent and the S&P 500 shed 0.69 per cent. Shares of industrial heavyweights Boeing and Caterpillar both fell, with Boeing down 1.7 per cent and Caterpillar down 2.3 per cent.
Tariffs of 25 per cent on steel imports and 10 per cent on aluminum will be imposed on the EU, Canada and Mexico from midnight (0400 GMT on Friday), Ross told reporters.
"We look forward to continued negotiations, both with Canada and Mexico on the one hand, and with the European Commission on the other hand, because there are other issues that we also need to get resolved," he said.
Canada and Mexico, embroiled in talks with the United States to modernize the North American Free Trade Agreement (NAFTA), responded swiftly.
Canada, the largest supplier of steel to the United States, will impose tariffs covering C$16.6 billion ($12.8 billion) on imports from the United States, including whiskey, orange juice, steel, aluminum and other products, foreign minister Chrystia Freeland said.
"The American administration has made a decision today that we deplore, and obviously is going to lead to retaliatory measures, as it must," prime minister Justin Trudeau said at a news conference with Freeland.