President Donald Trump signed orders imposing tariffs on steel and aluminum imports on Thursday. The tariffs will take effect in 15 days.

The president suggested earlier Thursday he would provide temporary exemptions for Canada and Mexico as he aims to revise the North American Free Trade Agreement.

Trump also suggested Australia and "other countries" might be spared. 

"The actions we're taking today are not a matter of choice, they're a matter of necessity for our security."

Trump says he's sticking with his initial plan for import tariffs of 25% on steel and 10% on aluminum. But the president says, "We're going to be very flexible."

"We're going to protect the American worker as I said I would do in my campaign," Trump explained during his Cabinet meeting on Thursday. He said that steel and aluminum were the "backbone of our nation" and the "bedrock of our defense and industrial base."

Trump said if a deal is reached with Canada and Mexico on NAFTA, "Most likely we won't be charging those two countries the tariffs." 

His comments in a Cabinet meeting come hours before a 3:30 p.m. meeting to formalize the tariffs amid uncertainty in the West Wing over which countries will be exempted from the protectionist measure.

Press secretary Sarah Huckabee Sanders told reporters on Wednesday that the exemptions would be on a "case by case" and a "country by country" basis.

The plan has roiled markets and drawn consternation from American allies, Republican free-trade advocates, and the business community.

Meanwhile, President Trump says the administration is asking China to trim the huge trade surplus it runs with the United States, the largest of any country.

The U.S. deficit with China hit a record $375 billion last year. Trump says Wednesday on Twitter that China has been asked to cut the surplus by $1 billion, or three-tenths of a percent.

Trump tweets: "China has been asked to develop a plan for the year of a One Billion Dollar reduction in their massive Trade Deficit with the United States." He says the U.S. relationship with China "has been a very good one, and we look forward to seeing what ideas they come back with. We must act soon!"

Trump says in a separate tweet that the U.S. is "acting swiftly" on intellectual property theft. U.S. Trade Representative Robert Lighthizer is overseeing an investigation into whether China is systematically violating U.S. intellectual property rights, particularly in the technology industry.

(The Associated Press, CBS News contributed to this report.)