US President Donald Trump has said a 25 per cent tariff on steel imports and 10 per cent on aluminium will come into effect in 15 days, but made no mention if Australia would be included.

In an announcement at the White House, Mr Trump also said Canada and Mexico will not initially be included, and that he was still working through which other countries may be excluded.

As he described the dumping of steel and aluminium in the United States as “an assault on our country”, Mr Trump told a news conference domestic production was needed for national security reasons.

He added the best outcome for all parties was for companies to move to the US.

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

Countries, including Australia, have 15 days to negotiate and apply for exemptions, according to the Trump administration.

Earlier, President Trump suggested that Australia would be among a select group of countries to be exempted from America’s proposed tariffs.

Calling Australia a “great country”, Mr Trump made his comments in a cabinet meeting Friday morning (AEDT) just hours before the formal resolution on the contentious trade measure.

The President said he’s sticking with his initial plan for import tariffs, but said the US is “going to be very flexible”.

“We have a very close relationship with Australia,” Mr Trump said.

“We have a trade surplus with Australia, great country, long term partner, we’ll be doing something with them.”

Along with an exemption for Australia, Mr Trump is expected to delay the imposition of tariffs on Canada and Mexico to increase pressure on negotiations over the North American Free Trade Agreement (NAFTA).

White House press secretary Sarah Huckabee-Sanders told reporters on Thursday that any exemptions would be on a “case by case” and a “country by country” basis.

Prime Minister Malcolm Turnbull had been lobbying Mr Trump in a bid to secure an exemption for Australia.

“I have been making personal representations to President Trump for some time,” Mr Turnbull told reporters in Sydney on Thursday.

Foreign Minister Julie Bishop is in New York and had been seeking talks with US Secretary of State Rex Tillerson to press Australia’s case for an exemption.

Thousands of Australian and US jobs could be affected if the proposed tariffs take in local exports.

The European Union has warned it could retaliate on US imports, including on products such as motorbikes, bourbon and jeans.

EU industry commissioner Elzbieta Bienkowska, who is visiting Australia, said it was important to send a strong message supporting trade liberalisation.

“We are for free trade, we don’t want to build walls around our territory,” she told AAP in Canberra on Wednesday night.

She said Europe was ready to confront the issue.

“We are prepared, as we were prepared for Brexit, we’re prepared for this,” Ms Bienkowska said.

-with AAP