President Donald Trump announced Tuesday the U.S. will pull out of the landmark nuclear accord with Iran, dealing a profound blow to U.S. allies and potentially deepening the president's isolation on the world stage.

"The United States does not make empty threats," he said from the White House in a televised address.

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. Iran has offered conflicting statements about what it may do — and the answer may depend on exactly how Trump exits the agreement.

Trump said he would move to reimpose all sanctions on Iran that had been lifted under the 2015 deal, not just the ones facing an immediate deadline. This had become known informally as the "nuclear option" because of the near certainty that such a move would scuttle the deal.

"At the heart of the Iran deal was a giant fiction. That a murderous regime desired only a peaceful nuclear energy program," Trump said. "Today, we have definitive proof that this Iranian promise was a lie."

He cited intelligence documents published last week by Israel, saying those documents "conclusively" showed Iran's "history of pursuing nuclear weapons. The fact is, this was a horrible, one-sided deal that should have never, ever been made."

Israeli Prime minister Benjamin Netanyahu gave a news conference on April 30 in which he presented information that he said showed that even after the deal, Iran 'continued to preserve and expand its nuclear weapons knowledge for future use.' (Amir Cohen/Reuters)

"It didn't bring calm, it didn't bring peace and it never will."

According to Reuters, most of the material the Israeli prime minister presented was from before the 2015 accord was finalized. Netanyahu did say, however, that Iran had added to its "nuclear weapons knowledge" since then.

What's next?

Supporters of fixing the agreement had hoped Trump would choose a piecemeal approach that could leave more room for him to reverse himself and stay in the deal if he could secure the additional restrictions that European nations tried unsuccessfully to negotiate with him.

Still, the administration planned to allow a grace period of at least three months and possibly up to six months so that businesses and governments can wind down operations that will violate the reimposed U.S. sanctions.

A woman walks past a mural on the wall of the former U.S. Embassy in Tehran on Tuesday. It's not yet clear how Trump's move will affect people in Iran. (Atta Kenare/AFP/Getty Images)

A slower withdrawal process could allow more room for Trump to reverse course later and decide to stay — if he secures the additional restrictions on Iran that European nations tried unsuccessfully to negotiate to prevent him from withdrawing.

Indeed, as administration officials briefed congressional leaders about Trump's plans Tuesday, they emphasized that just as with a major Asia trade deal and the Paris climate pact that Trump has abandoned, he remains open to renegotiating a better deal, one person briefed on the talks said.

Obama calls decision 'serious mistake'

Trump has lambasted the 2015 agreement, which was brokered by former president Barack Obama's administration, since his days as a presidential candidate.

On Tuesday, Obama said Trump's move was a "serious mistake" and "misguided," especially because Iran has been complying with the deal.

"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," Obama said.

There are few issues more important to the security of the US than the potential spread of nuclear weapons or the potential for even more destructive war in the Middle East. Today’s decision to put the JCPOA at risk is a serious mistake. My full statement: <a href="https://t.co/4oTdXESbxe">https://t.co/4oTdXESbxe</a> —@BarackObama

Without the deal, the U.S. "could eventually be left with a losing choice between a nuclear-armed Iran or another war in the Middle East," Obama said.

He added that the deal remains a model for what diplomacy can accomplish, including when it comes to North Korea.

Watch to see more of Trump's statement.

U.S. president says it was a 'horrible, one-sided deal' 0:45

Former U.S. secretary of state John Kerry, who played a key role in the Obama administration's effort to craft the deal, also said that withdrawing from the Iran nuclear deal "doesn't make sense."

Kerry, speaking at a gathering on food innovation in Milan on Tuesday, says he challenges anyone to find an agreement tougher than the one in place now.

United Nations Secretary General Antonio Guterres also called the deal a "major achievement in nuclear non-proliferation and diplomacy" and said he was "deeply disappointed" with Trump's decision.

Guterres called on all other UN member states to continue to support the Iran agreement.

France, Germany, and the UK regret the U.S. decision to leave the JCPOA. The nuclear non-proliferation regime is at stake. —@EmmanuelMacron

French President Emmanuel Macron tweeted that France, Germany and the U.K. "regret the U.S. decision" and that "the nuclear non-proliferation regime is at stake."

Macron said that they would work on a "broader" agreement "covering nuclear activity ... and stability in the Middle East, notably Syria, Yemen and Iraq."

Trump supporters argue with people protesting outside the White House as the president announced the withdrawal. (Brendan Smialowski/AFP/Getty Images)

Foreign Affairs Minister Chrystia Freeland on Tuesday reiterated Canada's support for the deal, known as the Joint Comprehensive Plan of Action (JCPOA).

"Iran must not be allowed to develop nuclear weapons," Freeland said in a statement, adding that the 2015 deal put Iran's nuclear program under a "rigorous and unprecedented international verification regime by the International Atomic Energy Agency."

Freeland's statement said while the deal isn't perfect, it has "helped to curb a real threat to international peace and security."

The foreign minister also noted that Canada has condemned Iran's ballistic missile program and maintains sanctions targeting it.

Iran, Israel react

Iranian President Hassan Rouhani reacted quickly to Trump's announcement in a live address on state television, saying there is a "short time" to negotiate with the countries remaining in the nuclear deal and he will be sending his foreign minister, Mohammad Javad Zarif, to meet with them.

Iranian President Hassan Rouhani had predicted problems for his country if the U.S. pulled out of the nuclear deal. After the news was confirmed, he said he'd try to talk to remaining nations, but didn't rule out future uranium enrichment. (Iranian Presidency Office via AP)

Rouhani warned Iran could start enriching uranium "in the next weeks."

Israeli Prime Minister Benjamin Netanyahu, meanwhile, praised Trump's decision, calling it a "historic move."

Netanyahu, a leading critic of the deal, said leaving it unchanged would be "a recipe for disaster, a disaster for our region, a disaster for the peace of the world."