Washington DC - President Donald Trump signed executive orders to impose 25% tariffs on steel and aluminum imports from March 12, ramping up a long-promised trade war despite warnings from Europe and China.

"Today, I'm simplifying our tariffs on steel and aluminum," Trump said Monday in the Oval Office. "It's 25% without exceptions or exemptions."

In an executive order released after, he said: "As of March 12, 2025, all imports of aluminum articles and derivative aluminum articles from Argentina, Australia, Canada, Mexico, EU countries, and the UK shall be subject to the additional ad valorem tariff."

Trump issued a separate order for steel, which said it would apply to all imports from the same countries the aluminum tariffs hit, as well as to Brazil, Japan, and South Korea.

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Canada – which Trump again said should be "our 51st state" – and Mexico are the biggest steel importers to the US, according to trade data. Brazil and South Korea are also major steel providers.

Trump also signaled he would look at imposing additional tariffs on cars, pharmaceuticals, and computer chips.

"President Trump has made it clear that an important part of an America First Golden Age is steel production," National Economic Council Director Kevin Hassett told CNBC.

Trump also promised an announcement on Tuesday or Wednesday on broader "reciprocal tariffs" to match the levies other governments charge on US products.

Canadian steelmakers warned of "massive" disruption, while the European Commission said it would "react to protect the interests of European businesses, workers and consumers from unjustified measures."

French President Emmanuel Macron vowed in an interview aired Sunday to go head-to-head with Trump over his wider tariff threats against the European Union, though he said the US should focus its efforts on China.

German economy minister Robert Habeck said a tariff conflict "only has losers."

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Around 25% of European steel exports go to the US, according to consultancy Roland Berger. Britain's steel industry body called the tariff plan a "devastating blow."

Trump also unleashed harsh tariffs on key trade partners China, Mexico, and Canada soon after he took office.

He paused 25% levies against Canada and Mexico for a month after both countries promised to step up measures to counter flows of the drug fentanyl and the crossing of undocumented migrants into the US.

But Trump went ahead with tariffs on China, the world's second-biggest economy, with products entering the US facing an additional 10% levy.

Chinese retaliatory tariffs targeting US coal and liquified natural gas came into play Monday.