The leaders of Germany, France and Britain, co-signers of the agreement, expressed regret and said they would try to salvage the accord with Iran. Iranian President Hassan Rouhani said he was sending his foreign minister to work with those remaining countries but warned there was only a short time to negotiate with them and his country could soon "start enriching uranium more than before."

The 2015 accord, which lifted major economic sanctions against Iran, was specifically aimed at preventing that result. But Trump said, "The Iran deal is defective at its core."

"If we do nothing, we know exactly what will happen. In just a short period of time, the world's leading state sponsor of terror will be on the cusp of acquiring the world's most dangerous weapons," Trump said in a televised address from the White House.

He said the United States "will be instituting the highest level of economic sanction."

Trump's decision means Iran's government must now decide whether to follow the U.S. and withdraw or try to salvage what's left of the deal. The leaders of Britain, Germany and France immediately urged the U.S. not to take any actions that could prevent them and Iran from continuing to implement the agreement. The statement from Prime Minister Theresa May, Chancellor Angela Merkel and President Emmanuel Macron also urged Iran to "show restraint" and continue fulfilling its own obligations such as cooperating with inspections.

In Washington, the Trump administration said it would re-impose sanctions on Iran immediately but allow grace periods for businesses to wind down activity.

The Treasury Department said there would be "certain 90-day and 180-day wind-down periods" but didn't specify which sanctions would fall under which timelines. Treasury said that at the end of those periods, the sanctions will be in "full effect."

National Security Adviser John Bolton said nobody should sign contracts for new business with Iran.

If the deal collapses entirely, Iran would be free to resume prohibited enrichment activities. Meanwhile, businesses and banks doing business with Iran will have to scramble to extricate themselves or run afoul of the U.S.

For nations contemplating striking their own sensitive deals with Trump, such as North Korea, the withdrawal will increase suspicions that they cannot expect lasting U.S. fidelity to international agreements it signs. Yet nations like Israel and Saudi Arabia that loathed the deal are likely to see it as a sign the United States is returning to a more skeptical, less trusting approach to dealing with adversaries.

Former President Barack Obama, whose administration negotiated the deal, called the Trump decision "misguided." He added that "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."

Trump, who repeatedly criticized the accord during his presidential campaign, said Tuesday that documents recently released by Israeli Prime Minister Benjamin Netanyahu showed Iran had attempted to develop a nuclear bomb in the previous decade, especially before 2003. Although Trump gave no explicit evidence that Iran violated the deal, he said Iran had clearly lied in the past and could not be trusted.

Iran has denied ever pursuing nuclear arms.

Netanyahu welcomed Trump's announcement, calling it a "historic move."

There was a predictably mixed reaction from Congress. Senate Majority Leader Mitch McConnell, a Kentucky Republican, said the Iran deal "was flawed from the beginning," and he looked forward to working with Trump on next steps. House Minority Leader Nancy Pelosi, a California Democrat, slammed Trump in a statement, saying this "rash decision isolates America, not Iran."