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."