The United States will double its tariffs on imported steel and aluminium starting on Wednesday, according to the White House, as it published an order signed by President Donald Trump.

The move marks a latest salvo in Trump's trade wars, bringing levies on both metals from 25 percent to 50 percent.

But tariffs on metal imports from the UK will remain at the 25 percent rate, while both sides work out duties and quotas in line with the terms of their earlier trade pact.

Trump announced his decision to hike tariffs on steel and aluminium when he addressed workers at a US Steel plant in Pennsylvania last week.

"Nobody is going to be able to steal your industry," he said at the time.

"At 25 percent, they can sort of get over that fence. At 50 percent, they can no longer get over the fence."

The move, however, fans tensions with key US trading partners.

The European Union warned over the weekend that it was prepared to retaliate.

It said the sudden announcement "undermines ongoing efforts to reach a negotiated solution" between the bloc and the United States.

Already, Washington is in talks with various countries after Trump imposed sweeping 10 percent tariffs on almost all partners in April and announced even higher rates for dozens of economies.

While the steeper levels have been paused during ongoing negotiations, this halt expires in early July – adding to urgency to reach trade deals.

Since returning to the presidency in January, Trump has imposed sweeping tariffs on allies and adversaries alike in moves that have shaken financial markets.

He has also imposed tariffs on sector-specific imports like autos, apart from targeting steel and aluminum.

Mexico will request an exemption from the higher tariff, Economy Minister Marcelo Ebrard said, arguing that it is unfair because the United States exports more steel to Mexico than it imports.

"It makes no sense to put a tariff on a product in which you have a surplus," Ebrard said.

Mexico is highly vulnerable to Trump's trade wars because 80 percent of its exports go to the United States, its main trading partner. (AFP)