Canada, Mexico and Europe had been exempted from import duties of 25 per cent on steel and 10 per cent on aluminum when they were first imposed in March, but those exemptions will expire as scheduled on Friday.
She added her voice to calls for British and European products to be exempt from the charges, saying they contributed to USA national security projects.
Canadians, industry and even opposition parties applauded the prime minister's unusually pointed rebuke of Trump and retaliatory tariffs.
Canadian Prime Minister Justin Trudeau, and several other world leaders, are not pulling any punches in the worldwide blowback to President Donald Trump's decision to put tariffs on steel and aluminum imports.
"These tariffs are an affront to the long-standing security partnership between Canada and the United States", said Trudeau, as Ottawa hit back with retaliatory duties on USA imports worth up to Can$16.6 billion (US$12.8 billion).
The European Union and China say they will deepen ties on trade and investment and that they fully support global trade rules, after U.S. President Donald slapped tariffs on steel and aluminum imports.
The Trump administration has demanded that Beijing make concessions and threatened to punish it for allegedly stealing USA technology by imposing tariffs on $50 billion of imports from China.
Juncker said the European Union, just as Mexico and Canada, would impose countermeasures-that is additional duties-on US imports.
Mexico struck back swiftly after the United States announced today that it would impose tariffs on Mexican metal imports.More news: Donald Trump wants to ban luxury German cars
More news: Macron talks to Trump, says tariffs unlawful and a mistake
More news: DxOMark: Xiaomi Mi 8 beats the Samsung Galaxy S9+ in still shots
"The US is playing a risky game here." said Cecilia Malmström, the European Union trade commissioner.
"We are not in a trade war, but we are in a very hard situation caused by the United States".
She added: "This is further weakening the transatlantic relations and it also increases the risk of severe turbulences in the markets globally". The European Commission said that it could take asymmetric measures, targeting classic American products such as Levi's jeans, Harley-Davidson motorbikes and bourbon whiskey.
Nevertheless, "the uncertainty about future trade and Trump's contempt for global rules can deal a significant blow to business confidence especially in trade-oriented nations", the expert said.
The Trump administration wants China to buy more U.S. goods to lower a trade deficit. He made the comment while outlining Canada's response to USA tariffs on steel and aluminum imports.
After trying to negotiate a resolution to softwood lumber and then NAFTA, the federal government had no choice but to retaliate, he said.
Some officials at the meeting said the tariffs made it harder for the group to work together to confront China over its trade practices.
Wang acknowledged the differences on trade issues but argued they were the result of the country's development process and advocated finding a consensual solution through dialogue.
Berenberg Bank economist Holger Schmieding argued that the direct impact of an US-EU trade war would actually be rather small in terms of gross domestic product. He wants to break up that system. "But we need to respond".
Germany's Volkswagen, Europe's largest automaker, warned that the decision could start a trade war that no side would win.