Opponents of the tariffs spent today engaged in last-minute lobbying to blunt the impact.

“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 during a Cabinet meeting.

More than $170 million of Australian steel and aluminium exports to the US could be at stake, largely through Melbourne-based company BlueScope Steel, which is the sole exporter of Australian steel to the US.

He reiterated that he would levy tariffs of 25 per cent on imported steel and 10 per cent on aluminium.

Mr Trump is expected to reveal in Washington DC at 3:30pm (7:30am AEDT) how his plan would work and which countries would be exempted.

White House officials said specifics of Trump’s plan remained fluid, and it was unclear when the tariffs would be finalised.

Administration officials have sought to iron out how certain national security “carve-outs” might be put in place to lessen the economic impact of the tariffs.

The process of announcing the penalties has been the subject of an intense debate and chaotic exchanges within the White House, pitting hardliners against free trade advocates such as outgoing economic adviser Gary Cohn aiming to add more flexibility for US trading partners.

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 speaking on the condition of anonymity to discuss internal thinking.

Steel and aluminium workers were invited to the White House for an afternoon meeting with Mr Trump, but it was unclear if the president plans to sign final orders or simply to discuss his pledge to impose the tariffs.

Mr Trump said on Twitter he looks forward to the meeting, tweeting, “We have to protect & build our Steel and Aluminum Industries while at the same time showing great flexibility and co-operation toward those that are real friends and treat us fairly on both trade and the military.”

Peter Navarro, Trump’s trade and manufacturing adviser, said in an interview on Fox Business that the tariffs would go into effect within about 15 to 30 days and that the proclamation signed by the president would include a clause that would not immediately impose tariffs on Canada and Mexico.

Press secretary Sarah Huckabee Sanders told reporters the exemptions would be made on a “case by case” and “country by country” basis, a reversal from the policy articulated by the White House just days ago that there would be no exemptions from Mr Trump’s plan.

The looming departure of White House economic adviser Gary Cohn, a former Goldman Sachs executive who has opposed the promised tariffs, set off anxiety among business leaders and investors worried about a potential trade war.

“We urge you to reconsider the idea of broad tariffs to avoid unintended negative consequences to the U.S. economy and its workers,” 107 House Republicans wrote in a letter to Mr Trump.

At the White House, officials were working to include language in the tariffs that would give Mr Trump the flexibility to approve exemptions for certain countries.

“He’s already indicated a degree of flexibility, I think a very sensible, very balanced degree of flexibility,” Commerce Secretary Wilbur Ross told CNBC. “We’re not trying to blow up the world.”

Mr Trump is expected to delay the imposition of tariffs on Canada and Mexico to turn up the pressure in negotiations over the North American Free Trade Agreement.