"These unilateral tariffs, imposed under a false pretext of safeguarding USA national security, are inconsistent with the United States' global trade obligations and WTO rules", Foreign Minister Chrystia Freeland said in a statement.
"This is a really sad time for us", Freeland said about the Trump administration's decision to extend steel and aluminium tariffs to Canada, the European Union and Mexico, all of which are close U.S. allies and have each announced plans to retaliate.
Canada's tariffs on imported US goods - everything from strawberry jam to sleeping bags - could steer consumers to seek out cheaper, made-in-Canada alternatives, but domestic industry players are fearful that input costs will rise and American politicians could retaliate in kind.
The statement, written by Canada, also called for "decisive action" to resolve the tariff dispute at a G7 leaders' summit starting on Friday in Charlevoix, Quebec.
Mr. Trump justified it as necessary to ensure American "national security" because restricting these imports under Section 232 of the U.S. Trade Expansion Act would expand the United States' own capacity to construct its own tanks and warships. The group includes Canada, France, Italy, Germany, the United Kingdom, Japan and the United States.
He further said that the retaliatory tariffs would only be applied to U.S. goods and would come into effect from July 1.
Kudlow defended the tariffs as a step to protect US interests, saying, "Trump is responding to several decades of trade abuses here".
"When you're nearly 800 billion dollars a year down on Trade, you can't lose a Trade War", he tweeted Saturday.
United States will continue to confront China over disputed islands, Mattis says
China dispatched heavy bombers to contested territories in the region in mid-May. Philip Davidson, the new head of U.S. Pacific Command, said in April.
The EU "will react firmly" to defend its interests against steep United States trade tariffs on steel and aluminium, the European Commission chief Jean-Claude Juncker said yesterday (1 March).
Trudeau said that the USA has a $2bn billion surplus on steel with the U.S., and was "very much aligned" on the issue of China.
Leaders of the group's member countries meet next week in Quebec and ministers urged the USA to abandon the tariffs ahead of the leaders' summit to prevent deeper divisions.
"I don't think our tariffs are anything to do with our friendship and our longstanding alliance with Canada", Kudlow said on the Fox News Sunday program.
His opinion was echoed by French Finance Minister Bruno Le Maire who said that Mnuchin was alone on the tariff issue, with the group becoming a "G6 plus one" as the other six G7 members had expressed "total incomprehension" over Washington's move.
Le Maire said it is up to the U.S. to take action to rebuild confidence among G7 members and to avoid any escalation during the upcoming leaders' summit. Trudeau said Thursday that the process imploded after Vice President Pence demanded that a renewed deal expire in five years.
Freeland said that the United States actions are "illegal under global trade laws". He exempted Canada, Mexico and the European Union pending additional talks to ease USA concerns. But Commerce Secretary Wilbur Ross said there was "no longer a very precise date" as to when talks would end and that the tariffs went into effect at midnight Thursday as a result.
Before last week's trade actions by the US, the European Union and Canada had been granted temporary exemptions.