Europe, Canada and Mexico are planning retaliatory moves after President Trump imposed tariffs on steel and aluminium imports to the US.
The European Union issued a 10-page list of tariffs on US goods ranging from Harley-Davidson motorcycles to food products.
It also plans to challenge the move at the World Trade Organization.
Mr Trump claimed the tariffs would protect US steelmakers, which were vital to national security.
French President Emmanuel Macron called Mr Trump to tell him the tariffs were “illegal” – a term echoed by Bernd Lange, chair of the European Parliament’s international trade committee.
The MEP hoped a trade war could be avoided but warned that Mr Trump’s action demonstrated the US president was “not willing to stick to the rules”.
Germany’s Economy Minister, Peter Altmaier, hoped a decisive EU response would make Mr Trump reconsider his decision.
UK International Trade Secretary Liam Fox said the 25% levy on steel was “patently absurd”, adding: “It would be a great pity if we ended up in a tit-for-tat trade dispute with our closest allies.”
Barry Gardiner, the Labour shadow trade secretary, told the BBC’s Today programme the US measures were “based on a lie”, adding the UK should not be “bullied by the president … we believe in a rules-based system and Trump doesn’t”.
Gareth Stace, head of trade body UK Steel, said the tariffs were “no way to treat your friend” and called on the government to safeguard the industry’s 31,000 jobs.
Justin Trudeau, the Canadian Prime Minister, said the US move was “totally unacceptable” and rejected the claim that his country posed a national security threat to America.
Canada plans to impose tariffs of up to 25% on about $13bn worth of US exports from 1 July. Goods affected will include some American steel, as well as consumer products such as yoghurt, whiskey and coffee.
Mexican Foreign Minister Luis Videgaray said his country was planning new duties for imports of steel, pork, apples, grapes, blueberries and cheese from the US.
Opposition to the tariffs was also voiced by prominent Republicans. House Speaker Paul Ryan, the most influential Republican in Congress, said the move “targets America’s allies when we should be working with them to address the unfair trading practices of countries like China”.
What do the US tariffs mean?
Mr Trump first announced plans for the tariffs in March, but granted some exemptions while countries negotiated.
On Thursday, US Commerce Secretary Wilbur Ross said talks with the EU, Canada and Mexico had not made enough progress to warrant a further reprieve, meaning tariffs of 25% on steel and 10% on aluminium have now come into effect.
They apply to items such as plated steel, slabs, coil, rolls of aluminium and tubes – raw materials that are used extensively across US manufacturing, construction and the oil industry.
Mr Ross said the president had the authority to lift the tariffs or alter them at any time, leaving room for “flexibility”.
“We continue to be quite willing and indeed eager to have discussions with all those parties,” he said.
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What will the economic effect be?
Canada, Mexico and the EU together exported $23bn (£17bn) worth of steel and aluminium to the US in 2017 – nearly half of the $48bn of total steel and aluminium imports last year.
European firms have said they fear lower US demand for foreign steel will divert shipments to Europe.
Analysts at IHS Markit expect the effects to be distributed across a wide range of markets, limiting the effect on steel prices outside the US.
That leaves America to bear the brunt of the economic impact, which economists say will appear in the form of higher prices and job losses – as many as 470,000 by one estimate.
Steel prices in the US have already risen due to the uncertainty and may increase as the tariffs hit imports.
Consumers outside the US could see prices of some goods fall, while those in America may end up paying more.