The plan has been widely criticised, including by prominent Republicans and White House advisers
US President Donald Trump has formally approved tariffs on imported steel and aluminium.
The US President has claimed the tariffs - 25% on steel imports and 10% on aluminum - are necessary to combat 'unfair trade practices' and to protect US national security.
They will come into force in 15 days’ time, but Canada and Mexico - who are currently renegotiating the North American Free Trade Agreement (NAFTA) with the US - will initially be exempt.
The White House confirmed that the two countries are considered a 'special case', and discussions will continue to 'address our concerns'.
Officials also say other individual countries could also be excluded from the new tariffs.
Looking forward to 3:30 P.M. meeting today at the White House. We have to protect & build our Steel and Aluminum 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.— Donald J. Trump (@realDonaldTrump) March 8, 2018
In a statement, the White House said: "Modification or removal of the tariffs would be possible if alternative means are agreed upon to ensure imports from a country no longer threaten to impair our national security."
US companies will also potentially be exempt, with the statement adding: "There will be a mechanism for US parties to apply for exclusion of specific products based on demand that is unmet by domestic production or on specific national security considerations".
Last week's announcement of the tariffs have provoked criticism both in the US and internationally, amid fears of a potential trade wars.
Such concerns were amplified when President Trump took to Twitter to insist: "When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win."
A spokesperson for senior Republican Paul Ryan said the House Speaker is "extremely worried about the consequences of a trade war and [is] urging the White House to not advance with this plan".
Trump's own economic adviser Gary Cohn announced his resignation earlier this week, apparently amid a disagreement with the president over the tariff plans.
Critics of the plan have suggested that other countries could impose retaliatory tariffs on American goods, and could negatively impact US industries reliant on imports.