The International Monetary Fund (IMF) has joined criticism of Donald Trump’s plan to impose a 25% tariff on steel imports and 10% 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.
EU trade chiefs are reportedly considering slapping 25% tariffs on around $3.5bn (£2.5bn) of imports from the US.
World Trade Organization Director General Roberto Azevedo said: “A trade war is in no one’s interests.”
But the rhetoric ramped up as Mr Trump tweeted, “Trade wars are good.”
Are trade wars good?
By Theo Leggett, business correspondent
If trade wars really were good and easy to win, the World Trade Organisation probably wouldn’t exist.
Most countries believe that negotiations are best carried out and disputes settled through a rules-based system. Introducing trade barriers on a tit-for-tat basis has the potential to harm companies on both sides.
But that’s unlikely to bother Mr Trump. His campaign rhetoric drew heavily on the perceived threat to traditional US industries from foreign interlopers acting unfairly. He’s simply continuing in that vein.
And it’s unlikely to register much with the steelworkers of Pennsylvania and Indiana. Concerned about their jobs and the future, many will welcome Mr Trump’s comments.
Read more at bbc.co.uk