US President Donald Trump's increased tariffs on steel and aluminum imports took effect on Wednesday as prior exemptions, duty free quotas and product exclusions expired.

His administration says the move is to reorder global trade norms in favour of America and its citizens.

Trump's action to bulk up protections for American steel and aluminum producers restores effective global tariffs of 25 percent on all imports of the metals and extends the duties to hundreds of downstream products made from the metals, from nuts and bolts to bulldozer blades and soda cans.

The runup to the tariff deadline came with some drama on Tuesday as Trump threatened Canada with doubling the duty to 50 percent on its steel and aluminum exports to America.

But Trump backed off those plans after Ontario Premier Doug Ford agreed to suspend his province's decision to impose a 25 percent surcharge on electricity exports to the states of Minnesota, Michigan and New York until earlier US tariffs were removed.

Ford said he would fly to Washington on Thursday with Canadian Finance Minister Dominic LeBlanc for talks with Commerce Secretary Howard Lutnick and other Trump officials to discuss revising the US-Mexico-Canada Agreement on trade.

The incident whip-sawed US financial markets already jittery over Trump's broad tariff offensive, but left unchanged Trump's original plans to strengthen the Section 232 national security tariffs on steel and aluminum imposed in 2018 during his first term.

A White House spokesperson described the US pressure on Canada as a "win" for the American people.

The US Customs and Border Protection agency cut off imports qualifying for duty-free entry under quota arrangements well before the midnight deadline, saying in a bulletin to shippers that quota paperwork needed to be processed by 4.30pm local time on Tuesday at US ports of entry or the full tariffs would be charged.

The move was welcomed by US steel producers as restoring Trump's original 2018 metals tariffs that had been weakened by numerous country exclusions and quotas and thousands of product-specific exclusions.

"By closing loopholes in the tariff that have been exploited for years, President Trump will again supercharge a steel industry that stands ready to rebuild America," Steel Manufacturers Association President Philip Bell said in a statement.

The countries most affected by the tariffs are Canada, the biggest foreign supplier of steel and aluminum to the US, Brazil, Mexico and South Korea, which all have enjoyed some level of exemptions or quotas. (Reuters)