CLYDE, UNITED STATES (AFP, REUTERS) - President Donald Trump on Thursday (Aug 6) announced he had ordered a 10 per cent tariff on Canadian aluminium be reimposed, saying America's key trading partner was flooding the US market with the metal.

"Canada was taking advantage of us, as usual," Trump said in a speech at a Whirlpool washing machine factory in Clyde, Ohio.

"I signed a proclamation that defends American industry by reimposing aluminium tariffs on Canada," he said.

Trump's order drew the ire of Canada, who pledged retaliation as tensions heightened between the close allies just weeks after the USMCA continental trade deal between the United States, Mexico and Canada came into effect.

Trump had exempted Canadian products from the tariffs as part of the USMCA on condition that they "not flood our country with exports and kill all of our aluminium jobs."

"Canadian aluminium producers have broken that commitment," he said.

The tariffs, which take effect Aug 16, are in response to what Washington called a "surge" in aluminium imports from Canada over the past year which "threatens to harm domestic aluminium production."

"I have determined that the measures agreed upon with Canada are not providing an effective alternative means to address the threatened impairment to our national security from imports of aluminium from Canada," Trump said in his proclamation.

Ottawa has long rejected the national security concerns coming from a close ally - the two nations have been joined in a free trade area since 1994.

Canadian Deputy Prime Minister Chrystia Freeland said the tariffs would hurt workers and regional economies already hit by the coronavirus pandemic and pledged Ottawa would retaliate as it had done in 2018, when Trump first imposed punitive measures on Canadian steel and aluminum.

"In response to the American tariffs, Canada intends to swiftly impose dollar-for-dollar countermeasures," Freeland said in a statement.

Freeland - in overall charge of relations with the United States - will formally respond to the tariffs at on Friday local time, her office said in a statement.

In 2018, Canada slapped tariffs on C$16.6 billion ($12.5 billion) worth of American goods ranging from bourbon to ketchup. Trump lifted the sanctions in 2019.

Canada's Prime Minister Justin Trudeau declined to travel to Washington to join in the USMCA launch ceremony early last month, after Trump floated the idea of again hitting the country with the tariffs.