Prime Minister Malcolm Turnbull says he will keep making Australia's case against steel tariffs with US President Donald Trump.

Key points: Spokeswoman Sarah Sanders says White House considering exemptions for Canada, Mexico and other countries

Spokeswoman Sarah Sanders says White House considering exemptions for Canada, Mexico and other countries Earlier today Julie Bishop said she had "no expectation" Australia would be spared

Earlier today Julie Bishop said she had "no expectation" Australia would be spared She has since noted a "change in language" from the White House

The proposed move to put a 25 per cent tariff on steel imports, and 10 per cent on aluminium, has sparked talk of a global trade war, should other economies retaliate with similar protectionist policies.

"I have been making personal representations to President Trump," Mr Turnbull said today.

He was reluctant to publicly say what he has said to the US administration about having Australia declared exempt, saying "megaphone diplomacy is rarely effective".

"I don't provide a running commentary on all of my advocacy with foreign governments, including the government of the US," he said.

"My job as Australia's chief advocate is to make our case persuasively and eloquently, or as eloquently as I can, but also to get the right results."

"In fact I discussed this issue of steel with him (Mr Trump) at some length just when I was in Washington a couple of weeks ago."

Exemptions for Canada, Mexico already on the table

Earlier today, President Trump's spokeswoman Sarah Sanders revealed the administration was considering exemptions for Canada, Mexico, and possibly other countries based on national security grounds.

"We expect that the President will sign something by the end of the week," Ms Sanders told a regular media briefing.

"There are potential carve-outs for Mexico and Canada based on national security, and possibly other countries as well based on that process."

Foreign Minister Julie Bishop said the Federal Government would renew its call to be exempt from the tariffs, given Mexico and Canada may be spared.

Analysis by chief foreign correspondent Philip Williams So are we or are we not? On the golden list of survivors … of the US steel and aluminium tariffs? Our government had pretty much given up on hopes we may be given a ticket of leave. But suddenly a ray of light emanating from Washington. The President's spokesperson Sarah Sanders says if it's seen in the nation's security interest some nations may be dropped off the list. Well we host the Pine Gap security establishment, and have a long and close history of shared military and security co-operation. Does that count? Certainly our diplomats will be working overtime to remind the Americans of a '100 years of mateship'. Not even our Foreign Minister said she believed we were in with a chance. But that was before the Sanders announcement. Maybe, just maybe, we can swing this. But it's a very long shot indeed.

Ms Bishop said the shift in language was a chance to reiterate the Federal Government's advocacy for Australian businesses, given steel and aluminium exports to the US were worth about $500 million a year.

"We will most certainly push our case even harder," Ms Bishop said.

"The Trump administration has only just confirmed there could be a country-by-country exemption and we will most certainly be putting our case, as we have been doing for the last couple of days.

"I note they mention Canada and Mexico, they are two countries that are currently negotiating with the US on the NAFTA agreement. We would work hard to ensure Australia's interests are taken into account."

Sorry, this video has expired Julie Bishop said earlier today Australia was unlikely to be exempt from the tariffs.

The internal White House dispute over tariffs contributed to the resignation of Mr Trump's chief economic adviser Gary Cohn.

White House officials said while the disagreement on tariffs was part of Mr Cohn's decision to resign, it was not the only reason.

Who would be most likely to receive exemptions?

Mr Turnbull and Trade Minister Steve Ciobo have also been lobbying to have Australia spared from the 25 per cent tariff on steel, and a 10 per cent tariff on aluminium imports.

Australia's major steel exporter BlueScope might get favourable treatment because it has a substantial presence in the United States and sends steel from Port Kembla to its manufacturing plant in California.

Compared to other countries arguing for exemptions, Australia sends far less aluminium and steel to the US each year.

According to two of the reports that informed the White House's decision to introduce tariffs, Canada is the top exporter of steel and aluminium to the US, sending billions of dollars worth of the metals to the US each year.

Those reports cite Mexico as the fourth-highest exporter of steel and 13th in aluminium. Australia is outside the top 20 in both steel and aluminium.

If any country is to be exempted from the tariff regime, the White House has made it clear that a consideration of US "national security" would be the key criterion.

In fact, the entire tariff hike the President is pursuing is based on the perceived dangers posed by "imports of steel [and aluminium] on the national security".

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The US takes a broad view of what it calls "the national security".

As defined by the Commerce Department, it includes "requirements for the US Department of Defence" but also encompasses "critical infrastructure sectors, including transportation systems, the electric power grid, water systems and energy generation systems".

The US defence force consumes millions of tonnes of steel and aluminium to build its fleets of ships and planes, but it is unlikely many countries would get tariff exemptions solely because of those naval and aerospace industry needs.

The needs of the US military account for only about 3 to 4 per cent of the country's total annual steel and aluminium consumption and its domestic mills and smelters have plenty of excess capacity to provide extra tonnages.

Any country anxious to secure a tariff exemption would struggle to win an argument purely because they are a supplier to the US' major defence contractors — they may be on slightly firmer ground if they resort to reminders of their loyalty and "mateship" to the US over many years.

Bishop warns of 'downward spiral' if countries retaliate

Ms Bishop said she had been in contact with US Secretary of State Rex Tillerson, but had not had a detailed discussion with him yet as he had been travelling.

She also warned the Trump administration's tariffs could provoke retaliations from other countries.

"Our concern and I think the concern being reflected across the world, is that there will be a reaction. There will be responses. And then you start the downward spiral," she said.

"What good has ever come from trade wars that result in a decrease in economic activity?"

This statement echoed a warning from the Reserve Bank yesterday that trade barriers could cause a "very big shock" for the world economy.

Reserve Bank governor Philip Lowe yesterday described the US plan as "highly regrettable" and "bad policy".

"This could turn very badly though if it escalates," he said.

"If we see retaliation and counter-retaliation, this could turn into a very big shock for the world economy."