The EU has launched its first counteroffensive against Washington's punishing steel and aluminium tariffs while the US began meetings in Canada with outraged finance ministers from its top trading partners.
Meanwhile, in Washington, Trump floated the possibility of scrapping the 24-year-old North American Free Trade Agreement in favour of separate bilateral deals with Canada and Mexico.
Brussels and Ottawa filed legal challenges at the World Trade Organization against Washington's decision. The EU, Canada, and Mexico also threatened stiff retaliatory tariffs as they pushed back against President Donald Trump's multi-front trade offensive.
British Prime Minister Theresa May said she was "deeply disappointed" and reiterated a call for Britain and the EU to be "permanently exempted" from the "unjustified" metals tariffs.
As the Group of Seven ministerial opened in Canada, US Treasury Secretary Steven Mnuchin faced stern reaction from his counterparts, who accused Trump of jeopardising the world economy with steps that would prove job killers for all concerned.
Canada Finance Minister Bill Morneau said the G7 discussions would be "difficult." "We are sending the message that these measures are not helpful," he told reporters.
And French Finance Minister Bruno Le Maire said Europe "will not negotiate with a gun to our heads." They are joined at the meeting by officials from Britain, Germany, Italy, and Japan.
"Unfortunately, we are being treated to a G6 + 1, with the United States squaring off against the rest and risking the economic destabilisation of the planet," Le Maire said.
The US imposed the tariffs in March, but gave Canada and the EU -- the biggest sources of foreign aluminium and steel for the US -- a grace period that ended at midnight Thursday.
The EU is preparing to slap tariffs on US products, including bourbon, motorcycles, and blue jeans, worth up to 2.8 billion euros ($3.3 billion). "If players in the world don't stick to the rulebook, the system might collapse. That is why we are challenging the US and China at the WTO," EU Trade Commissioner Cecilia Malmstrom said.
Macron told Trump in a telephone call that the tariffs were "illegal," and Merkel said the measure "risks touching off spirals of escalation that in the end hurt everyone." Canada unveiled a package of counter-tariffs on US imports valued at Can$16.6 billion (US$12.8 billion).
And Mexico said it would impose retaliatory duties on a variety of US goods, including steel and a host of agricultural goods, including pork, apples, and cheese.
The prospect of a global trade war has roiled financial markets this week even if they were back in positive territory on Friday due to upbeat US economic data.
Berenberg Bank economist Holger Schmieding argued that the direct impact of a US-EU trade war on the world economy would actually be rather small. Nevertheless, "Trump's contempt for international rules can deal a significant blow to business confidence especially in trade-oriented nations," Schmieding said. The WTO's former chief, Pascal Lamy, also said the damage would likely be limited in concrete terms.
"We have to keep things in proportion," he said on the French radio station France Info. He estimated that the economic impact of the tariffs would amount to "a very small part of trade flows as a whole." But others have estimated the impact as up to a full point off global growth, if the conflict expands and retaliation goes into effect.
The German carmakers' federation described the imposition of tariffs as "incomprehensible." "In a connected, global economy, customs barriers don't benefit anyone, including the United States," the VDA federation said.
Germany's carmakers are especially braced for the latest threat from Trump, who earlier this month launched proceedings that could eventually slap 25 per cent tariffs on auto imports.
"I know he has a very particular problem with German cars," warned Malmstrom.
US companies, too, have warned about the negative impact of the tariffs which will raise their costs.
"Plans by US manufacturers to expand will be put on hold indefinitely," said Paul Nathanson, a spokesman for the Coalition of American Metal Manufacturers and Users, adding, "Companies will be forced into difficult choices about technology, investment, and jobs."