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WASHINGTON — President Donald Trump on Thursday approved new steel and aluminum tariffs, capping off a week's worth of controversy over his plans to impose the trade penalties.

"Today, I'm defending America's national security by placing tariffs on foreign imports of steel," Trump said in the White House Roosevelt Room, flanked by steel and aluminum workers from Pennsylvania and Kentucky, shortly before he signed proclamations imposing the tariffs. "You don't have steel, you don't have a country."

The tariffs — 25 percent on imported steel and 10 percent on imported aluminum — will take effect March 23, a senior administration official told reporters.

Mexico and Canada were granted immediate exemptions from the tariffs because of the ongoing NAFTA renegotiation talks, Trump said. There has been no specific timeline laid out by the administration for how long those exceptions might last.

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"I have a feeling we'll make a deal on NAFTA," Trump said. If that happens, "there won't be any tariffs" on Canada or Mexico. On Thursday morning, Trump told reporters as he wrapped up a Cabinet meeting that the policy toward longtime trade partners, like Australia, would be "flexible."

Earlier, the senior administration official said that "all countries will be welcome to discuss" other possible ways to ease the tariffs. As far as other countries that might seek exemptions, Trump said at the afternoon announcement that in the coming weeks "we'll see who's treating us fairly and who's not treating us fairly."

Trump added, "Many of the countries that treat us the worst on trade and on military are our allies, as they like to call them."

For those countries that don't want to pay import duties on their products, the president offered advice: "Bring your plant to the U.S.A. There's no tax."

While the White House has repeatedly said the tariffs should come as no surprise, the news has riled many GOP lawmakers, U.S. trade partners, and even some inside Trump's White House. Gary Cohn announced his resignation as National Economic Council director on Tuesday after the president's unexpected announcement that he planned to enact the trade penalties.

Top Republicans were still in open disagreement with the leader of their party when the president put pen to paper to authorize the tariffs. At Thursday's announcement, Trump was joined by U.S. workers from the affected industries — but not GOP congressional leaders.

"I disagree with this action and fear its unintended consequences," House Speaker Paul Ryan said in a statement. Sen. Jeff Flake, R-Ariz., a frequent Trump detractor, immediately introduced a bill to nullify the tariffs, calling them "a marriage of two lethal poisons to economic growth — protectionism and uncertainty" and warning that they would lead to "economic disaster."

Sen. Ben Sasse, R-Neb., said in a statement that "temporary exceptions for Canada and Mexico are encouraging but bad policy is still bad policy, and these constant NAFTA threats are nuts."

"We're on the verge of a painful and stupid trade war, and that's bad," he added.

But a senior administration official fired back Thursday at the "lobbyists and the politicians and the swamp creatures," ripping their concerns about protectionism and trade wars as "all fake news."