US President Donald Trump has signed off on deeply contentious trade tariffs, swatting aside warnings of a global trade war and protests from allies in Europe and at home.
Mr Trump has approved levies of 25% on imported steel and 10% on imported aluminum.
The White House said the measures would come into effect in 15 days.
The tariffs will not apply to Canada and Mexico initially, with officials saying that security and trade partners could negotiate to avoid the measures coming into effect.
Mr Trump said: "We have to protect and build our steel and aluminium industries while at the same time showing great flexibility and cooperation toward those that are real friends and treat us fairly on both trade and the military."
Describing the dumping of steel and aluminum in the US as "an assault on our country," Mr Trump later said that the best outcome would be for companies to move to the US.
He insisted that domestic production was needed for national security reasons.
"If you don't want to pay tax, bring your plant to the USA," he said.
Mr Trump has come under pressure from members of his own Republican party.
Tonight, Speaker Paul Ryan said he feared there could be "unintended consequences" after the introduction of the tariffs.
"We will continue to urge the administration to narrow this policy so that it is focused only on those countries and practices that violate trade law," he said.
Senator Jeff Flake said he would introduce a law to nullify the tariffs, while Republican chairman of the Senate Finance Committee, Orrin Hatch, also criticised the tariffs, but said he would work with the White House to "mitigate the damage".
Analysts believe the plan to offer relief to Canada and Mexico is a gesture aimed at putting pressure on them to give ground in separate talks on the North American Free Trade Agreement (NAFTA), which appear to be stalled.
US stocks rose after the announcement, while stock markets in Canada and Mexico rallied on the news, as did the Canadian dollar and the Mexican peso.
Mexico has rejected any linkage to NAFTA in robust terms, with Mexican Economy Minister Ildefonso Guajardo saying: "Under no circumstance will we be subject to any type of pressure."
Canadian Trade Minister Francois-Phillippe Champagne said his country would not accept any duties or quotas from the US.
Mr Trump has also demanded concession from the EU, complaining that it treated American cars unfairly and has threatened to hike tariffs on car imports from Europe.
The tariffs have triggered the threat of countermeasures from the EU and China. They aim to hit China, although it exports very little of either metal to the United States.
Several major trading partners have said they will respond to the tariffs with direct action.
European Financial Affairs Commissioner Pierre Moscovici had warned that the EU had a "whole arsenal at our disposal with which to respond".
Countermeasures would include European tariffs on US oranges, tobacco and bourbon, he said.
Harley Davidson Inc motorcycles have also been mentioned, targeting Speaker Ryan's home state of Wisconsin.