The tariffs add to mounting worries over a trade war as the Trump administration also considers tariffs on US auto imports - which could hit top suppliers from Mexico, Canada, Japan and Germany - and plans to levy duties on $50 billion in Chinese goods.
"We are prepared to react in a united and clear way whatever the decision of the (US) president", he said.
"This is protectionism, pure and simple", said Jean-Claude Juncker, president of the European Commission.
The European Union has already notified the World Trade Organization of plans to levy duties on $7.2 billion worth of USA exports in response, with the aim of collecting $1.6 billion in tariff revenue.
Europe had previously outlined a list of items, including USA bourbon, cranberries and jeans, as potential targets for retaliation.
Mexico announced in a statement Thursday it would retaliate against the U.S. by imposing "equivalent measures" on products such as "flat steel, lamps, pork and numerous produce items until the United States eliminates the imposed tariffs".
Donald Trump's long-standing promise to use import tariffs to try to revive the United States steel and aluminum industry was opposed by U.S. economists, labor experts, and even the industries themselves-but the White House did it anyway.
USA crude oil futures settled 1.7 per cent lower at $67.04 a barrel despite a larger-than-expected decline in inventories. Thursday's tariff announcement by the White House will further complicate agreeing on a G-7 agenda.
However, after early negotiations, the European Union was able to negotiate a temporary exemption until June 1.
The tariffs were initially announced earlier this year but Brussels, which negotiates trade matters on the UK's behalf, won temporary exemptions that were due to expire today.
Oil and gas industry representatives are expressing worries about new tariffs the Trump administration is imposing on steel and aluminum imports from three key US allies.
And he said Trump has the authority to alter the tariffs or impose quotas or "do anything he wishes at any point" - allowing "potential flexibility" to resolve the issue.
"We regret that. We would much rather move together in partnership", he said.
However, his decision was met with strong criticism including from Republicans, which in part lead to him to delay the tariff on the EU, Mexico and Canada, key USA allies, until June 1. It goes through that to the supply chain into the steel industry.
"The steel and aluminum tariffs will have a poisonous effect on the NAFTA negotiations", Alden said.
Shares of United States steel producers gained in trading on Thursday, while companies that rely on the metals, such as Caterpillar and Boeing, declined.
"That Canada could be considered a national-security threat to the United States is inconceivable", said Trudeau, adding that the people of the USA are not Canada's target, and that the federal government would have far preferred that its hand wasn't forced.
"Trade wars don't have any winners".
Steel and aluminum exports are significant to Canada's economy. Steffen Seibert, a German government spokesperson, also called the tariffs "unlawful".
The tariffs are part of Trump's effort to protect USA industry and workers from what he described as unfair global competition, a key theme of his "America First" agenda.
"Now is not the time for any disproportionate escalation, and we urge the European Union to consider this when initiating its response", he added.
Initial local reaction to President Donald Trump's 25 per cent tariffs on Canadian-made steel is swift and overwhelmingly critical. Like Trump, Hatch is a member of the Republican Party.
He said: "The administration will need to come to Capitol Hill to provide answers about the indiscriminate harm these tariffs are causing our local businesses".
Prime Minister Justin Trudeau slammed U.S. President Donald Trump's decision to slap tariffs on Canadian steel, calling the move "totally unacceptable". He said that talks with Canada and Mexico over amending the North American Free Trade Agreement were "taking longer than we had hoped".
"We urge the European Union to consider this when initiating its response".
Jean-Baptiste Lemoyne, junior minister of foreign affairs, told journalists in Paris Thursday: "France disapproves of these unjustified and unjustifiable measures".
Senate Finance Committee chairman Orrin Hatch described the tariffs as "a tax hike on Americans" that will have damaging consequences for consumers, manufacturers and workers alike.
A list of countermeasures would include tariffs on USA metals and food products such as yogurt, pizza and cucumbers, as well as household products like mattresses and refrigerators.
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