The US is to impose tariffs on steel and aluminium imports from key allies in Europe and North America.
The US said a 25% tax on steel and 10% tax on aluminium from the EU, Mexico and Canada will start at midnight.
The move immediately triggered vows of retaliation from Mexico, Canada and the EU, which called the tariffs "protectionism, pure and simple".
The UK said it was "deeply disappointed" by the US decision, which followed weeks of negotiations.
EU trade commissioner Cecilia Malmstrom said it was a "bad day for world trade", while European Commission president Jean-Claude Juncker said the move was "totally unacceptable".
The EU has "no choice" but to bring a case before the World Trade Organization and impose duties on US imports, he added.
Canadian Prime Minister Justin Trudeau said the tariffs were an "affront" to the longstanding relationship between Canada and the US, especially to the "thousands of Canadian soldiers who fought and died alongside their American comrades-in-arms" in Afghanistan.
He said Canada plans to levy tariffs on American products worth about $13bn starting 1 July. It is also planning a challenge at the World Trade Organization.
He said: "We have to believe that at some point common sense will prevail but we see no sign of that in this [US] action today."
What American products will be affected by retaliation?
Canada said it would put 25% tariffs on certain types of American steel, as well as a 10% tax on other items, including yoghurt, whiskey and roasted coffee.
Mexico's Economy Ministry said it is planning new duties for steel, pork legs and shoulders, apples, grapes, blueberries and cheese.
Europe had previously outlined a list of items, including US bourbon, cranberries and jeans, as targets for retaliation.
How did this start?
US President Donald Trump announced plans for tariffs on foreign steel and aluminium in March, justifying them on national security grounds.
He has argued that global oversupply of steel and aluminium, driven by China, threatens American steel and aluminium producers, which are vital to the US.
Since the announcement, South Korea, Argentina, Australia and Brazil have agreed to put limits on the volume of metals they can ship to the US in lieu of tariffs.
The US granted temporary exemptions to the EU, Canada and Mexico amid negotiations over limits. That deadline was due to expire on 1 June, having already been extended by a month.
What did the US announce?
US Commerce Secretary Wilbur Ross announced the sanctions from Paris, where he had been negotiating with EU leaders who were trying to avert the tariffs.
Ross said talks had not made enough progress to warrant a further reprieve either for Europe or for Canada and Mexico, which are in negotiations with the US over North America's free trade agreement.
Trump has the authority to lift the tariffs or alter them at any time, he added.
"We continue to be quite willing and indeed eager to have discussions with all those parties," Ross said.
The Trump administration is determined to wring concessions from its trading partners around the world - and it's prepared to do so in belligerent style.
The President believes cheap imports harm US industry and cost US workers their jobs. On the campaign trail, he promised them a fairer deal - and it seems he's happy to risk a trade war to get what he wants. In fact, he has previously suggested that trade wars are "good, and easy to win".
So will it go that far? Certainly, the EU and Mexico seem prepared to retaliate - and Canada has hinted it in the past might take action as well. So it's not looking good. But if you look at Washington's dealings with China - its key target on trade - things seem less clear cut.
On several occasions the rhetoric between the two sides has been ramped up, threats have been made, then positions have softened. So far, an all-out trade war has been avoided. And China has made some concessions.
So perhaps this is simply how negotiations take place in the Trump era. Loudly - and in a blaze of publicity.
What does this do to the market?
Canada, Mexico and the EU combined exported $23bn worth of steel and aluminium to the US last year - nearly half of the $48bn total steel and aluminium imports in 2017.
Since Trump's announcement in March, companies in the US that buy metals have already reported higher steel prices and and complained that US producers do not have the ability to meet demand.
Economists and US companies have warned that higher metal costs will disrupt supply chains and eventually get passed on to US households. They said the tariffs are likely to lead to job losses at firms that rely on the materials.
Goldman Sachs in March estimated that the tariffs would boost inflation by less than half a percentage point.
Ross has dismissed the concerns about higher costs, arguing that the effects will be minimal.
Shares of US steel producers gained in trading on Thursday, while companies that rely on the metals, such as Caterpillar and Boeing, declined.
UK producers, like those in mainland Europe, are worried the tariffs will cause a decline in US demand, while prompting a surge in steel imports diverted from the US.
The disruption would hit the industry hard, said Gareth Stace, director of trade association UK Steel.
The UK steel industry employs about 31,000 people and sends about 7% of its exports to the US, according to the organisation.
What is the reaction in the US?
The tariffs have faced sharp criticism in the US from businesses and lawmakers, including Republicans typically allied with the president.
Republican Kevin Brady, who shepherded last year's tax cut through Congress and chairs the House Ways and Means Committee, called on the administration to reinstate the exemptions.
He said: "The administration will need to come to Capitol Hill to provide answers about the indiscriminate harm these tariffs are causing our local businesses."
The US Aluminum Association, which represents major producers, also criticised the US decision, saying tariffs would alienate allies and fail to address oversupply.
What happens next?
France's junior trade minister Jean-Baptiste Lemoyne said he expects EU counter-measures to be completed by mid-June.
Canada's tariffs - which target about $13bn worth of US products - are expected to start on 1 July.
China has already imposed duties on $3bn worth of US goods, including wine and nuts, in retaliation for the steel and aluminium tariffs.
Europe, Canada and Mexico say that as close allies of the US they should receive exemptions from the measures.
Trudeau said: "It is simply ridiculous to view any trade with Canada as a national security threat to the US."
The UK said: "We will continue to work closely with the EU and US Administration to achieve a permanent exemption, and to ensure that UK workers are protected and safeguarded."