At the same time, Prime Minister Justin Trudeau warned that the proposed US levies would cause significant pain on both sides of the border.
"If Canada fails to obtain an exemption from unjust and punitive tariffs on steel and aluminum exports to the US, then the Canadian government has no choice but to withdraw from NAFTA renegotiations," Unifor said in a statement.
The union represents tens of thousands of workers in the automotive manufacturing, steel and aluminum sectors across Canada.
"It's clear the US is using tariffs as a trade weapon," said Unifor's Renaud Gagne. "It's no accident that these duties were announced while NAFTA talks are underway.
"The federal government must stand and fight, here and now, against this threat by the US government and supporting stakeholders," he said.
Ottawa did not directly respond to Unifor's call, but Trudeau said: "The United States has a $2 billion surplus on steel with us, so we regard the imposition of any tariffs on steel and aluminum between our two countries as absolutely unacceptable."
Due to the high level of integration of the Canada-US steel industries, he added, "these proposals are going to hurt them every bit as much as they will hurt us."
US President Donald Trump's sudden announcement on Thursday that his administration plans to impose duties of 25 and 10 percent on imports of steel and aluminum, respectively, comes as a seventh round of talks aimed at revamping the North American Free Trade Agreement (NAFTA) is underway in Mexico.
It also follows recent US trade attacks on other major Canadian sectors including forestry and aircraft manufacturing.
Despite the latest salvo, a senior Canadian official said NAFTA discussions "over the last four days have actually been very productive and a whole bunch of chapters in NAFTA have been closed."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)