Donald Trump has announced steep tariffs on imports of steel and aluminium into the US to address what he says is an “assault on our country.”

The president said a 25% tax will apply to steel imports, and 10% will be added to aluminium brought into the US.

Mr Trump said the excess of imported steel and aluminium was a “travesty” and hurts American workers and industry.

Making the long-awaited announcement on Thursday, the president said the industry had been “ravaged by aggressive foreign trade practices”.

“It’s really an assault on our country,” he said.

Sources outside the White House said the tariffs will take effect in 15 days, with Canada and Mexico indefinitely excluded from the duties.

Those briefed on the plans say all countries affected by the tariffs are being invited to negotiate for exemptions, if they can address the threat their exports pose to the US.

American steel and aluminium workers have long been betrayed, but “that betrayal is now over,” Mr Trump said. The former real estate developer said politicians had for years lamented the decline in the industries, but nobody was willing to take action.

As he has indicated previously, Mr Trump confirmed he would levy tariffs of 25% on imported steel and 10% on aluminium. But he said during a Cabinet meeting earlier in the day that the penalties would “have a right to go up or down depending on the country and I’ll have a right to drop out countries or add countries. I just want fairness”.

Business leaders, meanwhile, have continued to sound the alarm about the potential economic fallout from tariffs, with the president and CEO of the US Chamber of Commerce raising the spectre of a global trade war. That scenario, Tom Donohue said, would endanger the economic momentum from the GOP tax cuts and Trump’s rollback of regulations.

“We urge the administration to take this risk seriously,” Mr Donohue said.

The president suggested in the meeting with his Cabinet that Australia and “other countries” might also be spared, a shift that could soften the international blow amid threats of retaliation by trading partners.

“We’re going to be very fair, we’re going to be very flexible but we’re going to protect the American worker as I said I would do in my campaign,” Mr Trump said.

People briefed on the plans ahead of the announcement said all countries affected by the tariffs would be invited to negotiate with the administration to be exempted from the tariffs if they can address the threat their exports pose to US manufacturers.

The process of announcing the penalties has been the subject of an intense debate and chaotic exchanges within the White House, pitting hard-liners against free trade advocates such as outgoing economic adviser Gary Cohn.

The fight over tariffs comes amid intense turmoil in the West Wing, which has seen waves of departures and negative news stories that have left Mr Trump increasingly isolated in the Oval Office, according to two senior officials.

Congressional Republicans and business groups are bracing for the impact of the tariffs and the departure of Mr Cohn, a former Goldman Sachs executive who has opposed them.

House Speaker Paul Ryan, appearing at a session with Home Depot employees in Atlanta, said ahead of Mr Trump’s announcement: “I’m just not a fan of broad-based, across-the-board tariffs.” He pointed to the store’s many products that rely on steel and aluminium.

More than 100 House Republicans wrote to Mr Trump on Wednesday, asking him to reconsider “the idea of broad tariffs to avoid unintended negative consequences” to the US economy and workers.

Arizona Senator Jeff Flake, a Republican, said he plans to introduce legislation next week to nullify the tariffs though he has acknowledged that finding the votes to stop the president’s actions could be difficult.

The president has said the tariffs are needed to reinforce lagging American steel and aluminium industries and protect national security.