Canada will also launch challenges of the US tariffs at the World Trade Organization and through a Nafta panel.
He added they had "made some progress", but not enough to warrant a new exemption.
Tariffs of 25% on cars and auto parts, as Mr Trump apparently wants, would be disastrous for Canada's and Mexico's vehicle industries, though American buyers of cars and parts would suffer too.
Italy's FTSE MIB is now down 0.13 per cent, while Spain's Ibex is 0.2 per cent lower and Germany's Dax is down 0.75 per cent.
"The responsibility falls exclusively on to U.S. authorities", Le Maire said.
Financial markets sank on concerns about the disputes among trading partners, with the Dow Jones industrial average falling more than 200 points. Accordingly, despite the historical precedent for unilateral, protectionist trade measures to result in a weaker US Dollar, market participants are seemingly unmoved by the latest developments. "This is protectionism, pure and simple", Juncker said.
European officials, bracing for the tariffs, have threatened to retaliate against US orange juice, peanut butter, kitchenware, clothing and footwear, washing machines, textiles, whiskey, motorcycles, boats and batteries.
"By targeting those who are not responsible for overcapacities, the U.S.is playing into the hands of those who are responsible for the problem", he said. United States leaves us no choice but to proceed with a WTO dispute settlement case and the imposition of additional duties on a number of U.S. imports.
Commenting on the exchange, commissioner for trade Cecilia Malmström said: "Today is a bad day for world trade".
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The UK said: "We will continue to work closely with the European Union and US Administration to achieve a permanent exemption, and to ensure that UK workers are protected and safeguarded".
"First of all, those who will suffer most are the poorest, the less privileged people, those who actually rely on imported goods to have their living", she said, adding that longstanding supply chains also would be disrupted.
German Chancellor Angela Merkel said the European Union would give a "smart, determined and jointly agreed" response to the new tariffs that she said would break WTO rules.
The EU had already threatened reprisal tariffs, which could tip the United States into another trade war.
Unsurprisingly, the countries hit by the tariffs are fighting back. The price of hot-rolled steel has risen above US$890 per tonne, up from $745 per tonne prior to the tariff announcement and almost double the price before Trump was elected.
An official statement from the European Commission says it is determined to shield the EU steel and aluminium markets from damage caused by additional imports that might be coming into the EU as a result of the closure of the U.S. market.
The decision will most likely lead to retaliation from the three United States allies.
In a statement released by Ontario NDP leader and Hamilton Centre MPP, Andrea Horwath said the announcement will immediately impose devastating tariffs on Canadian steel and aluminum - putting many thousands of good jobs in Ontario at risk. And Arkansas-based Big River Steel is considering a $1.5 billion facility expansion at an existing manufacturing plant in its home state of Arkansas, according to reports.
France's finance minister, Bruno Le Maire, called the USA tariffs "unjustified, unjustifiable and risky".