WASHINGTON — President Donald Trump announced Tuesday that he is withdrawing the United States from the Iran nuclear deal, reneging on a landmark pact and raising fears that Tehran might respond by resuming its frozen weapons program.

"It is clear to me that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement," Trump said in a nationally televised statement from the White House. "The United States will withdraw from the Iran nuclear deal."

Trump further said that he would impose the "highest level" of sanctions, which would affect not only Iran but other countries that do business with it. The Treasury Department said a series of primary and secondary sanctions — those affecting American and foreign partners of Iran — would go back into effect after wind-down periods specified by law.

In 2015, Iran agreed to halt its pursuit of nuclear weapons — and to allow for international checks on its facilities — in exchange for moves by the U.S., several other countries and the United Nations to roll back sanctions that had crippled its economy. The accord was widely seen as the biggest foreign policy accomplishment of President Barack Obama's administration.

But Trump has long been a critic of the agreement — calling it the "worst deal ever" — maintaining that Obama gave up too much for too little. His decision signals a dramatic reversal of U.S. policy, and his rhetoric was hawkish.

The move also raises two important questions for the Trump administration and the international community: Whether Iran will respond by resuming its quest for nuclear weapons — and whether reneging on the Iran pact might affect North Korea's willingness to cut a denuclearization deal of its own with the president.

U.S. Secretary of State Mike Pompeo was in the air on his way to North Korea during the announcement, Trump said. Critics of Tuesday's move said Trump is giving North Korea little reason to trust him.

"In a democracy, there will always be changes in policies and priorities from one Administration to the next," Obama said in a statement Tuesday calling Trump's actions "misguided" and harmful to America's standing in the world. "But the consistent flouting of agreements that our country is a party to risks eroding America's credibility, and puts us at odds with the world's major powers."

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Meanwhile, French President Emanuel Macron, who spoke with Trump Tuesday morning before the announcement, tweeted his "regret" over the president's decision. "The nuclear non-proliferation regime is at stake," he wrote.

The withdrawal of the U.S. leaves a big hole at the negotiating table after Trump had sought over the past six months to force a new deal by threatening to scrap the existing one.

Still, some in the administration and outside it were hopeful that his declaration would create pressure on Iran and America's partners in that deal — Britain, France, Germany, Russia and China — to negotiate a new agreement in which Iran abandons its ballistic missile development, funding of terrorist groups, stated quest to destroy Israel and cyber-attacks on the U.S. and its allies.

That group included some American critics of Trump's move, including members of his own party, who expressed hope that the U.S. would be a part of a renegotiated agreement in the coming months.

"Without proof that Iran is in violation of the agreement, it is a mistake to fully withdraw from this deal," said Rep. Mike Turner, R-Ohio, a senior member of the House Armed Services Committee. "Now, we need to work with our allies to fix this flawed agreement to ensure the world is not facing a nuclear Iran.”

For his part, Iranian President Hassan Rouhani said Tuesday that his country would remain engaged with the other signatories to the original deal.

"We've known for months that President Trump wouldn't be committed to this deal," he said. "The Iranian foreign ministry will continue talks with remaining countries in the deal."

It is difficult to say exactly how Trump’s maneuver is likely to play out over time. The U.S. sanctions are complex, and only part of the international picture. Moreover, the reactions of each country involved in the original deal are difficult to predict precisely — including that of Iran itself: Even if Tehran doesn’t kick-start its nuclear weapons program, its leaders could banish inspectors.

In the meantime, foreign countries and companies will be risking their ability to do business in the U.S. if they continue to buy Iranian oil.

Iran initially came to the negotiating table because of the pain caused by economic sanctions imposed by Congress, a series of U.S. presidents, several foreign nations and the United Nations.

Proponents of the deal say that it has worked — that Iran has kept its promises to halt the nuclear weapons program and to allow international vetting of its energy capabilities.

But Trump campaigned on rewriting the pact, and in a speech to the U.N. last year, called it a major "embarrassment" and "one of the worst and most one-sided transactions the United States has ever entered into."

Until Tuesday, though, he had taken a more measured approach to the deal. Last October, in declining to certify that Iran had lived up to its end of the bargain, Trump kicked a decision on whether to re-impose U.S. sanctions to Congress — which did not act.

Trump pushed the call to Congress, even though he always has had the power to re-impose sanctions through executive action and perhaps because he knew it was unlikely Congress would be capable of agreeing to anything.

Back then, his national security adviser was H.R. McMaster, who was considered a moderate on Iran. Now, though — with leading Iran critic John Bolton newly installed in McMaster's old role — Trump has decided to take on Tehran, a decision that has worried America's European allies.

Trump's announcement did draw a positive response from some, including many congressional Republicans and the Saudi government — whose U.S. ambassador tweeted that it "fully supports the measures" — and Israeli Prime Minister Benjamin Netanyahu, who gave a much-publicized presentation late last month in which he accused Iran of violating the deal because it hadn't shared pre-existing plans for nuclear weapons with the international community.

In his remarks Tuesday, Trump pointed to thousands of documents detailing the program that Israeli intelligence operatives had spirited from Iran. In a televised address following Trump's speech, Netanyahu praised the president for his "courageous leadership."