The International Monetary Fund has joined criticism of Donald Trump's plan to impose a 25 per cent tariff on steel imports and 10 per cent on aluminium.
The body warned that such a move would hurt the US as well as other countries.
It said others could follow the US president's precedent by claiming tough trade restrictions were needed to defend national security.
Canada, the largest supplier of steel to the US, said tariffs would cause disruption on both sides of the border.
It is one of several countries that have said they will consider retaliatory steps if the president presses ahead with his plan next week.
World Trade Organization Director General Roberto Azevedo said: ‘A trade war is in no one's interests.’
But the rhetoric ramped up as Trump tweeted, ‘Trade wars are good.’
Critics argue that the tariffs would fail to protect American jobs and ultimately raise prices for consumers.
But US Commerce Secretary Wilbur Ross used a can of Campbell's Soup to defend the tariffs on Friday morning as ‘no big deal’.
He told CNBC the duties would have a negligible effect on the price of a tin, amounting to less than a cent.
‘Who in the world is going to be too bothered?’ he asked.
Trump has lamented the decline of the US steel industry, which since 2000 has seen production drop from 112m tons to 86.5m tons in 2016.
The number of employees working in the sector has fallen over the same period from 135,000 to 83,600.
But experts say far more Americans work in industries that depend on steel products than are employed in steel plants.
Steel mills in 2015 employed about 140,000 Americans, according to census data.
But 6.5 million Americans work for manufacturers who make things using steel.
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