"We are imposing dollar for dollar tariffs for every dollar levied against Canadians by the US," he wrote.

He added Canada would challenge the "illegal" measures under North American Free Trade Agreement and the World Trade Organisation rules, and finished by taking a swipe at Trump's leadership style.

"We have to believe that at some point their common sense will prevail. But we see no sign of that in this action today by the US administration."

A 25 per cent tariff on steel imports and 10 per cent tariff on aluminium imports would be imposed on the EU, Canada and Mexico starting at midnight local time, Ross told reporters.

Canada would then impose retaliatory tariffs on $C16.6 billion ($16.93 billion) worth of US exports and those will stay in place until the US lifts its own measures, Foreign Minister Chrystia Freeland said.

"The American administration has made a decision today that we deplore and obviously is going to lead to retaliatory measures, as it must," a visibly annoyed Trudeau said at a news conference with Freeland on Thursday.

"We regret that. We would much rather move together in partnership."

Trudeau said that in NAFTA talks, Canada, the US and Mexico had come so close to a deal he had offered to meet Trump in Washington.

But he said US Vice-President Mike Pence told him on Tuesday that as a precondition for that meeting, Trudeau would have to agree to a five-year sunset clause. He refused.

"There was the broad lines of a decent win-win-win deal on the table," Trudeau said.

In Europe, Juncker promised "counter-balancing measures" soon.

"It's totally unacceptable that a country is imposing unilateral measures when it comes to world trade," he said.

The EU, Mexico and Canada had initially received exemptions when Trump declared global tariffs on imported metals, which started in March.

Those temporary waivers were extended through May while the White House sought negotiated concessions.

The EU has previously warned of potential retaliation if Washington imposed permanent tariffs.

Brussels submitted a list to the WTO of US products that could be slapped with punitive import duties.

"What they can do, we are able to do exactly the same," Juncker said.

The EU list covers agricultural products including corn, rice, peanut butter and orange juice.

Whiskey, tobacco and denim jeans would also be subject to higher tariffs, as well as various steel products, motorcycles and boats.

The German and French governments issued a joint statement saying they supported counter-measures.

Macron called the tariffs "illegal" and a "mistake", and planned to speak with Trump later on Thursday, local time.

He said the US President's decision was a mistake because it created economic and commercial nationalism.

He ominously recalled the pre-World War II period saying, "economic nationalism leads to war. This is exactly what happened in the 1930s."

A spokesman for German Chancellor Angela Merkel warned of "the danger of spirals of escalations" that could hurt everyone.

The British government said it was "deeply disappointed" and the UK and other European Union countries "should be permanently and fully exempted" from the tariffs.

The Mexican government, already at loggerheads with Trump over immigration and the border wall, said it would levy import taxes on US exports of pork bellies, blueberries, apples, grapes, certain cheeses, and various types of steel.

Reuters, AP, Fairfax Media, agencies