US President Donald Trump on Thursday signed off on controversial tariffs on steel and aluminium, which will come into effect in 15 days.

Canada and Mexico will be exempt from the levies of 25% on imported steel and 10% on imported aluminium, while there is the possibility of modifying or removing the tariffs for other countries.

"A strong steel and aluminum industry are vital to our national security," Trump said.

"You don't have steel, you don't have a country."

Trump’s sudden push for the tariffs last week triggered fears of a global trade war and rattled financial markets.

Plans for the tariffs, set to start in 15 days, have stirred opposition from business leaders and prominent members of Trump's own Republican Party, who fear the duties could spark retaliation from other countries and hurt the US economy.

Within minutes of the announcement, Republican Senator Jeff Flake, a Trump critic, said he would introduce a bill to nullify the tariffs.

But some Democrats praised the move, including Senator Joe Manchin of West Virginia, who said it was "past time to defend our interests, our security and our workers in the global economy and that is exactly what the president is proposing with these tariffs."

European Union Trade Commissioner Cecilia Malmstrom said: "The EU is a close ally of the US and we continue to be of the view that the EU should be excluded from these measures. I will seek more clarity on this issue in the days to come."

Meanwhile, US steel-consuming industries sharply criticised the tariffs as damaging to their prospects.

"The US will become an island of high steel prices that will result in our customers simply sourcing our products from our overseas competitors and importing them into the United States tariff-free," the Precision Metalforming and National Tooling And Machining associations said in a joint statement.

Several major trading partners have said they might respond to the tariffs with direct action.

Countermeasures could include European Union tariffs on US oranges, tobacco and bourbon. Harley-Davidson Inc motorcycles have also been mentioned, targeting Republican US House of Representatives Speaker Paul Ryan's home state of Wisconsin.

Even as Trump threatened tariffs and prodded his NAFTA partners, 11 nations gathered in Chile to sign a landmark Asia-Pacific trade pact, one that Trump withdrew from on his first day in office.