US President Donald Trump has officially announced steep tariffs on imported steel and aluminum, but exempted Canada and Mexico.

Defying his own party and delivering on a campaign promise to fight unfair practices by America’s trading partners, Trump signed paperwork enacting tariffs of 25 percent on steel and 10 percent on aluminum during a ceremony at the White House.

 "Today, I am defending America’s national security by placing tariffs on foreign imports of steel and aluminum," said Trump who was flanked by steel and aluminum workers.

Trump said the American steel and aluminum industry has been “ravaged by aggressive foreign trade practices.”

He described the dumping of steel and aluminum in the United States as “an assault on our country.”

“If you don’t want to pay tax, bring your plant to the USA,” he said.

The president temporarily exempted the countries from the tariffs  that “treat us fairly on trade,” a move aimed at putting pressure on Canada and Mexico to give ground in separate talks on the North American Free Trade Agreement (NAFTA). Trump has separately discussed withdrawing the United States from NAFTA.

He has also asked the European Union to offer concessions, arguing that it treated American cars unfairly and has threatened to hike tariffs on auto imports from Europe.

The World Trade Organization (WTO) director-general has called on Trump not to impose hefty tariffs on aluminum and steel exports, saying it could cause a deep global recession.

"An eye for an eye will leave us all blind, and the world in deep recession," Roberto Azevedo told members of his organization on Monday. We "must make every effort to avoid the fall of the first dominoes."

America’s top energy industry trade groups, which rely on imported steel for drilling equipment, pipelines, liquefied natural gas terminals and refineries, issued statements calling on Trump to scrap the plan.

Trump is also facing resistance from politicians in his own party. Republicans have lobbied the president to reconsider his plans, but he has brushed off such demands saying, “We're not backing down” on steel tariffs.