Trump authorises tariffs on steel, aluminium imports

US President Donald Trump on Thursday signed an order imposing stiff and sweeping new tariffs on imported steel and aluminum, defying protests at home and abroad and raising the spectre of trade wars.

The metals tariff order that Trump signed at the White House authorises 25 per cent duty on imported steel and a 10-per cent duty on imported aluminum, in order to safeguard national security and to stop the ''assault on our country''.

The United States is the largest steel importer in the world and the order could hit South Korea, China, Japan, Germany, Turkey and Brazil the hardest.

The Presidential order, signed at the White House on Thursday, however, sought to soften the impact on the United States' closest allies with a more flexible plan than originally envisioned.

Trump's tariff order temporarily exempts two key trading partners, Canada and Mexico, and keeps the door open for carve-outs for other countries like Australia. Trump said the order would temporarily exempt Canada and Mexico, pending discussions with both about the terms of trade, including already tense talks over the North American Free Trade Agreement.

Officials from Canada and Mexico, however, said they will not be bullied into accepting a Nafta deal that could disadvantage their countries.

But foreign leaders warned of a trade war that could escalate to other industries and take aim at American goods.

''The actions we are taking today are not a matter of choice; they are a matter of necessity for our security,'' Trump said after signing the tariff order, which will go into effect in 15 days.

Flanked by a handful of steel and aluminum workers, some wearing factory uniforms and holding hard hats, Trump presented the move as a way to rebuild vital industries hit hard by foreign competition. ''Our factories were left to rot and to rust all over the place; thriving communities turned into ghost towns,'' he said. ''That betrayal is now over.''

The orders that Trump signed on Thursday goes against the spirit of free market that has largely governed Washington under previous administrations – both Republican and Democratic – and are destined to rewrite the rules of global trade. Trump, a longtime critic of globalisation, argued that the United States had been ravaged by unfair trading partners.

The White House said any nation with a security relationship with the United States was welcome to discuss ''alternative ways to address the threatened impairment of the national security caused by imports from that country.'' Those talks could result in the tariff being lifted, the order said.

Trump said Robert Lighthizer, the top United States trade negotiator, would be in charge of negotiating with countries asking for exemptions in the next 15 days.

''We look forward to educating the Trump administration on the vital role the Japanese steel industry plays in the American marketplace,'' said Tadaaki Yamaguchi, a steel executive and the chairman of the Japan Steel Information Center. ''The Japanese industry is not part of the import problem but a solution.''

Wilbur Ross, the commerce secretary, will lead a parallel process that could result in the exemption of certain products made of steel and aluminum that American companies need, but that are not manufactured domestically.

Products as varied as construction cranes and railroad ties are made with specialised steel that is not available widely, if at all, from United States manufacturers.

During a cabinet meeting earlier in the day, Trump singled out Australia as an example of another country that could be excluded, citing the trade surplus that the United States maintains with Australia, which imports more from America than it exports to the country.

The announced trade barriers came just hours after a group of countries signed the Trans-Pacific Partnership, a sweeping trade deal that no longer includes the United States. Trump, a fervent opponent of the deal, officially withdrew the United States from it on his fourth day in office.