European Commission chief Jean-Claude Juncker vowed the bloc "will react firmly" to defend its interests.
A federal government official has told CBC News on background that Canada believes any new tariffs should not apply to this country, due to the highly integrated nature of the North American steel market, and because of the close co-operation between the two countries on defence issues.
His administration has since made it clear the tariffs - due to be announced formally this week - would apply to imports from any country, including Britain. "One problem with exclusions is that it's a slippery slope. This is now going to give us an opportunity to potentially reopen facilities and grow new business", Frabotta said.
China has already threatened to restrict imports of USA soybeans, although the country accounts for just 2 percent of US steel imports. "We'll see. The president makes the decisions", he said.
Navarro said on Sunday that the tariffs were set to be signed by the President this week or next.
The union represents tens of thousands of workers in the automotive manufacturing, steel and aluminum sectors across Canada.
The European Union has said it plans to apply 25% tariffs on $3.5 billion of United States imports to the continent in response to "rebalance" trade. We are the third-largest producer of steel in the world already.
Trump frequently singled out China for criticism during his election campaign, saying the trade relationship was "the greatest theft in the history of the world" and comparing it to "rape".
"The United States has an $800 Billion Dollar Yearly Trade Deficit because of our "very stupid" trade deals and policies", Trump said in a tweet earlier in the day.
Trudeau said Trump's plan will have "significant and serious" economic ramifications on both sides of the Canada-US border, Xinhua reported.
He later followed this up with further tweets, saying the U.S. must "protect our country and our workers" and that the trade deficit left him with no choice on tariffs.