Iranian President Hassan Rouhani has ordered the country's atomic agency to prepare to enrich uranium to "industrial levels" and warned Donald Trump that he will not be able allowed to win the "psychological war" with his country.
Speaking on the state-run television station, Mr Rouhani said he had ordered his foreign ministers to negotiate with governments besides the US.
He said there is a "short time" for the deal to survive without Washington involved and said should those negotiations fail, there will be "industrial level" uranium enrichment for use in nuclear weapons.
Mr Trump has long criticised the six-party nuclear deal signed in 2015, signed under his predecessor Barack Obama, and has repeatedly promised to withdraw the US from it.
Announcing his decision to do so, he said that Iran was a "regime of great terror" and that "no action taken by the regime has been more dangerous than its pursuit of nuclear weapons and the means of delivering them".
Prime Minister Theresa May, French President Emmanuel Macron and German Chancellor Angela Merkel issued a joint statement saying they regretted his decision, calling it a threat to global efforts to contain nuclear weapons.
Mr Macron tweeted that the "nuclear non-proliferation regime is at stake" because of the announcement.
The other two parties to the deal - China and Russia - have not weighed in as yet to this historic shift. However, last month Russian Foreign Minister Sergei Lavrov said that his country vowed to stop any attempts by the US to "sabotage" the deal.
European Union foreign policy chief Frederica Mogherini said the Iran nuclear agreement is a pillar of international security and called on the remaining parties to continue to honour the terms.
She said in a statement, directed towards Mr Rouhani: "Do not let anyone dismantle this agreement."
Although the Trump administration agreed Iran’s nuclear programme is a major threat, the president announced in October 2017 he would not re-certify a nuclear deal signed by Iran and six world powers.
Despite the evidence provided by the United Nations on Tehran's compliance with the deal, Mr Trump said it was too lenient on Iran and has maintained the idea that the country has violated portions of it.
Not re-certifying the deal or abandoning it will open the door for harsher economic sanctions to be placed on the country, the mitigation of which was a key inducement for Iran to comply with the historic deal. What concerned many Washington insiders - and today's announcement may have proven them right - is that Mr Trump's newest hire as National Security Adviser John Bolton has been quite hawkish on pulling out of the deal as is new Secretary of State Mike Pompeo, who has called the agreement "disastrous".
The Trump administration said it will reimpose nuclear sanctions on Iran immediately, but allow grace periods for businesses to wind down activity so they do not violate the sanctions. The Treasury Department said there will be "certain 90-day and 180-day wind-down periods" but has not specified which sanctions will fall under which timelines but indicated that at the end of those periods, the sanctions would be in "full effect".
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