President Donald Trump has announced the withdrawal of the United States from the Iran nuclear deal.
Speaking at the White House, he also announced the re-establishment of US sanctions against Iran.
The decision is likely to raise the risk of conflict in the Middle East, upset America's European allies and disrupt global oil supplies.
Mr Trump described the deal as "defective at its core," and said that he was ready, willing and able to negotiate a new deal with Iran when it is ready.
"I am announcing today that the United States will withdraw from the Iran nuclear deal," Mr Trump said at the White House.
"In a few moments, I will sign a presidential memorandum to begin reinstating US nuclear sanctions on the Iranian regime. We will be instituting the highest level of economic sanctions."
He called Tehran the world's leading state sponsor of terror, and decried its influence in the Middle East.
The 2015 deal, worked out by the United States, five other international powers and Iran, eased sanctions on Iran in exchange for Tehran limiting its nuclear programme.
The Joint Comprehensive Plan of Action (JCPOA) was signed following lengthy negotiations.
Mr Trump said the agreement, the signature foreign policy achievement of his predecessor Barack Obama, does not address Iran's ballistic missile programme, its nuclear activities beyond 2025, nor its role in conflicts in Yemen and Syria.
Iranian President Hassan Rouhani has said that his country remains committed to the agreement.
"If we achieve the deal's goals in cooperation with other members of the deal, it will remain in place ... By exiting the deal, America has officially undermined its commitment to an international treaty," Mr Rouhani said in a televised speech.
"I have ordered the foreign ministry to negotiate with the European countries, China and Russia in coming weeks. If at the end of this short period we conclude that we can fully benefit from the JCPOA with the cooperation of all countries, the deal would remain," he added.
Mr Trump's move is a snub to European allies such as France, Britain and Germany who are also part of the deal, and tried hard to convince the US President to preserve it.
France, Germany and Britain have said that they are committed to implementing the 2015 deal, despite Mr Trump's decision to pull out and his threat of sanctions.
"Our governments remain committed to ensuring the agreement is upheld, and will work with all the remaining parties to the deal to ensure this remains the case including through ensuring the continuing economic benefits to the Iranian people that are linked to the agreement," they said in a joint statement.
United Nations Secretary-General Antonio Guterres has called on the remaining parties in the Iran deal to abide by the deal.
In a statement, Mr Guterres said that he was "deeply concerned" by Mr Trump's decision.
"It is essential that all concerns regarding the implementation of the plan be addressed through the mechanisms established in the JCPOA. Issues not directly related to the JCPOA should be addressed without prejudice to preserving the agreement and its accomplishments."
Mr Trump did not provide details of the economic sanctions that he is reimposing on Iran.
However, he implied that he was going beyond not renewing waivers on sanctions related to Iran’s oil exports and its central bank that were due to expire on Saturday, and reimpose all of the other US sanctions that were suspended under the nuclear deal.
Former US President Barack Obama described Mr Trump's decision as "misguided" and a "serious mistake".
In a statement, Mr Obama said: "The reality is clear. The JCPOA is working.
"That is a view shared by our European allies, independent experts, and the current US secretary of defense."
"That is why today's announcement is so misguided. I believe that the decision to put the JCPOA at risk without any Iranian violation of the deal is a serious mistake."
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: https://t.co/4oTdXESbxe— Barack Obama (@BarackObama) May 8, 2018
Tánaiste and Minister for Foreign Affairs and Trade Simon Coveney said he was "greatly disappointed" the US is withdrawing from the nuclear agreement.
In a statement, Mr Coveney said: "we share many of the concerns which the US has expressed about other aspects of Iranian policy, but the way to address these is not to move away from the one area where significant positive progress has been made. That remains our view, and I hope that the United States will reconsider this decision.
"I hope that all other parties to the agreement, including Iran but also the EU and others, will continue to implement the agreement.
"The Middle East, and the world, are safer and more stable with this agreement in operation."