Washington (CNN) President Donald Trump announced Tuesday he is quitting the Iran nuclear deal, pitting him against the United States' closest allies and leaving the future of Tehran's nuclear ambitions in question.

"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 from the White House Diplomatic Room. "The Iran deal is defective at its core. If we do nothing we know exactly what will happen."

In announcing his long-telegraphed decision, Trump said he would initiate new sanctions on the regime, crippling the touchstone agreement negotiated by his predecessor. Trump said any country that helps Iran obtain nuclear weapons would also be "strongly sanctioned."

"This was a horrible one-sided deal that should have never, ever been made," the President said in remarks that, at times, misrepresented the international agreement's provisions . "It didn't bring calm, it didn't bring peace, and it never will."

Trump's decision could have explosive consequences, straining longstanding US alliances, disrupting oil markets and boosting tensions in the Middle East, even if the US reversal doesn't lead Iran to restart its atomic program.

While Trump supporters praised the move, analysts and critics said it undermines Washington's credibility in future negotiations -- particularly with North Korea -- and potentially empowers the very hardliners in Iran that Trump vilified in his remarks.

It also further isolates Trump on the global stage, where he has angered even the staunchest US allies by reneging on US commitments to the Paris climate accord and pulling out of the Trans-Pacific Partnership trade agreement.

Reaction was swift

Former President Barack Obama , who rarely comments on his successor, issued a statement describing Trump's move as a "serious mistake" that could leave the US with a "losing choice between a nuclear-armed Iran or another war in the Middle East."

Some of the US' closest allies, the UK, France and Germany, issued a statement expressing "regret and concern" about the decision, emphasizing Iran's compliance with the deal and their "continuing commitment" to the Joint Commission Plan of Action, as the deal is formally known.

Iran's President, Hassan Rouhani, said he had ordered the country's atomic industry to be ready to restart industrial uranium enrichment, while the country's foreign minister said he would work with the pact's remaining partners -- France, the UK, Germany, China and Russia -- to see whether they could ensure "full benefits for Iran. Outcome will determine our response," Javad Zarif tweeted.

Tensions in the region are high, with US officials citing "concerns" that Iran might attack Israel, without citing their evidence for the claim, while Israel called up reserves and the State Department issued a security alert for US citizens in the Golan Heights.

Turkish President Recep Tayyip Erdogan told CNN that he fears that "new crises" will break out in the Middle East as a result of the US decision.

"We don't need new crises in the region," Erdogan told CNN's Becky Anderson.

US foes used the decision to portray the US as an international outlier, underscoring that the US, not Iran, is now technically in violation of the deal.

"The position promulgated by Washington represents a significant violation of the JCPOA," the Russian foreign ministry said in a statement, describing Trump's decision as "new confirmation of Washington's incompetence."

The Russians also said that US "claims regarding Iran's absolutely legitimate nuclear activities are just a cover for keeping political scores with the country."

Indeed, senior Trump administration officials -- including Secretary of State Mike Pompeo and the Director of National Intelligence Daniel Coats -- have said Iran is adhering to its commitments under the deal.

But Trump has argued while they may be sticking to the letter of the accord, they have violated its spirit by fostering discord in the region, supporting groups like Hezbollah, Houthi rebels in Yemen, and the Syrian regime.

Trump derided the deal as an embarrassment that gave the regime dollars at the same time it sponsored terrorism.

"At the point when the US had maximum leverage, this disastrous deal gave this regime -- and it's a regime of great terror -- many billions of dollars, some of it in actually cash -- a great embarrassment to me as a citizen," Trump said.

Lawmakers, such as Pennsylvania Republican Sen. Pat Toomey, backed the decision.

"I support US withdrawal from the flawed, dangerous Iran nuclear deal," Toomey said, calling for the US to develop harsh new sanctions to punish Iran for "its grotesque human rights abuses, openly hostile aggression in the Middle East, extensive ballistic missile testing, and support for terrorism."

Sanctions don't go into effect for months

The sanctions could take months to go into effect as the US government develops guidance for companies and banks. But reapplying the sanctions -- which were lifted in exchange for Iran's commitment to curb its nuclear program -- will effectively cripple the 2015 accord that Trump has deemed a disaster.

The grace period could allow for further negotiations between US allies on a side agreement that addresses Trump's concerns about the missile program and Tehran's support for terror groups. Trump said Tuesday he was open to finding diplomatic means to address his concerns.

But even if a deal is struck, it's not clear how they would convince Iran to sign on, or whether Russia and China -- two other partners to the deal -- would agree.

New US sanctions will undoubtedly cause companies to reconsider investments in Iran and European firms may have no choice but to scale back or risk running afoul of US rules.

Questions surrounding the announcement also highlighted a seeming lack of strategic planning by the Trump administration.

US officials admit that during negotiations with European officials about a side deal to address Trump's concerns about missiles and Iran's regional activities, they did not discuss what they would do if the US walked away from the deal.

No Plan B

"We did not talk about a Plan B because we were focused on negotiating a supplemental agreement," a senior State Department official said Tuesday, "so we did not -- we did not talk about Plan B."

That official said the US is prepared to sanction Europeans with the end goal of creating a "global coalition" to work toward a comprehensive agreement that addresses all US concerns about Iran using the leverage of economic isolation.

It's a plan that has former officials scratching their heads.

"It's very difficult to see how we get a better deal given that Iran would have no reason to go back to the negotiating table and no reason to trust us," said Andrew Keller, a former deputy assistant secretary of state for sanctions and counter threat finance.

"Even if we were to get the Europeans to agree to something, what is the administration's plan to get the Russians on board, the Chinese on board, not to mention the Iranians," Keller said. "And if there's not a plan for a new and better deal, how can we possibly be safer outside of this one?"

The State Department official said the administration doesn't know whether or to what degree Europeans will be on board -- discussions with European allies began this afternoon, the official said -- nor how Iran will react.

National security adviser John Bolton, an Iran hawk who nonetheless told people he was committed to providing all options to Trump, offered a variety of paths, including reimposing all sanctions, applying new sanctions, or allowing for more time to negotiate with the Europeans.

Trump determined that more time would not bridge disagreements, most pointedly his demand that Iran's nuclear program be curbed past the current deal's sunset in 2030.

Some White House officials were caught off guard when Trump announced on Twitter Monday that he was planning to make the announcement. Most aides expected him to wait until closer to Saturday, when the deadline for issuing a sanctions waiver landed.

One US official suggested the timing was moved up so as not to impede next week's opening of the US embassy in Jerusalem, another move that could cause jitters in the Middle East.