The United States President, Donald Trump, announced on 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.

“I am announcing today that the United States will withdraw from the Iran nuclear deal,” Trump said from the White House.

“It is clear to me that we cannot prevent an Iranian nuclear bomb under the decaying and cotton structure of the current agreement,” he said. “The Iran deal is defective at its core. If we do nothing, we know exactly what will happen.”

“The so-called Iran deal was supposed to protect the United States and our allies from the lunacy of an Iranian nuclear bomb, a weapon that will only endanger the survival of the Iranian regime,” the President said. “In fact, the deal allowed Iran to continue enriching uranium and, over time, reach the brink of a nuclear breakout.”

The President added: “Today, we have definitive proof that this Iranian promise was a lie.”

Long a harsh critic of the nuclear accord, Trump has until now resisted taking steps to fully withdraw from the plan.

The decision 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.

Other signatories to the plan, including Germany, France and the UK, have said they would maintain their commitments.

The grace period until the sanctions are imposed may offer the deal's proponents an opening to negotiate on a side agreement that addresses Trump's concerns about the missile program and Tehran's support for terror groups. 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.

The sanctions Trump is expected to allow to move forward target Iran's oil exports. The status of additional actions remains unclear. But new US sanctions would 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.