French President Emmanuel Macron has said the United States' decision to impose tariffs on European, Canadian and Mexican metals exports was illegal and a mistake.
The French leader said he would talk to President Donald Trump on the subject later this evening.
Canadian Prime Minister Justin Trudeau described the tariffs as totally unacceptable, with the country set to announce retaliatory measures.
Mexico said it would continue serious negotiations with the US despite what it called "unjust and unilateral" tariff measures.
It was confirmed earlier that the US would impose tariffs on steel and aluminium imports from the European Union, Canada and Mexico from midnight.
It ended months of uncertainty over potential exemptions and sharply escalating the risk of a trade war.
US Commerce Secretary Wilbur Ross told reporters in a telephone briefing that the US would proceed with plans for a 25% tariff on steel and a 10% tariff on aluminium imports.
However, he said the door was still open for negotiations without specifying what measures could be taken.
"We look forward to continued negotiations, both with Canada and Mexico on the one hand, and with the European Commission on the other hand, because there are other issues that we also need to get resolved," Mr Ross said.
The tariffs, which have prompted several challenges at the WTO, are aimed at allowing the US steel and aluminium industries to increase their capacity utilisation rates above 80% for the first time in years.
President Trump's administration has threatened to impose tariffs on car imports, is engaged in negotiations with China to reduce America's yawning trade deficit and has said it will punish Beijing for stealing its technology by imposing tariffs on $50 billion of imports from China.
After months in which it appeared the Trump administration had been backing away from tariffs amid in fighting between the president's top economic advisers, the US has over the past week ramped up its threats on trade.
German magazine Wirtschaftswoche reported that Mr Trump had told Mr Macron that he wanted to stick to his trade policy long enough that Mercedes-Benz cars were no longer cruising through New York.
The Trump administration launched a national security investigation last week into car and truck imports, using the same 1962 law that he has applied to curb incoming steel and aluminium.
France's Finance Minister Bruno Le Maire had met Mr Ross today in an attempt to end the stand-off over steel and aluminium, a move that ultimately failed to sway the US administration.
"It's entirely up to US authorities whether they want to enter into a trade conflict with their biggest partner, Europe," Mr Le Maire told reporters after the meeting.
Europe did not want a trade war, he said, but Washington had to back down from "unjustified, unjustifiable and dangerous tariffs".
The European Union would respond with "all necessary measures" if the United States imposed them.
The European Commission, which coordinates trade policy for the 28 EU members, has said the bloc should be permanently exempted from the tariffs since it is an ally and not the cause of steel and aluminium overcapacity.
The EU has threatened to retaliate with tariffs on Harley Davidson motorcycles and bourbon, measures aimed at the political bases of Republican legislators who they wanted to stand up to Mr Trump.
German Chancellor Angela Merkel said the European Union would give a "smart, determined and jointly agreed" response to new US tariffs that she said would break World Trade Organisation rules.