Prime Minister Jacinda Ardern said the US withdrawal from the Iran nuclear agreement was a step backwards.

"Our view is that it made for a more stable, predictable Middle East. So the decision today is disappointing."

"The world is better off having a deal rather than no deal."

She added: "What we need to focus on is how we have a rules-based system that gives the stability and the security that the things like the Iran deal have provided.

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"There is no point criticising it for what it didn't do when that may not have been the original intent of the deal.

Foreign Minister Winston Peters is reserving judgment on US President Donald Trump's decision to unilaterally pull his country out of the landmark nuclear accord.

Peters told Newstalk ZB's Mike Hosking he hoped Trump and his advisers were right.

"I have been around long enough to think, let's hope you are right. Let's keep an open mind here. Because if you are wrong the world will be so much worse off for it."

Foreign Minister Winston Peters said he hoped US President Trump was making the right decision. Photo / Mark Mitchell
Foreign Minister Winston Peters said he hoped US President Trump was making the right decision. Photo / Mark Mitchell

He couldn't say whether or not it was the right decision.

"I can't say this far away. We are a country which is very much inclined to have people rush to judgment, and push politicians to make comment when simply they and their advisers do not have the information."

Trump said this morning the 2015 deal, which included Germany, France and Britain, had failed and Iran had continued to build its nuclear weapons programme.

Trump said there was definitive proof Iran has lied about its nuclear activities, had funded global terrorism by plundering the wealth of its own citizens.

Keeping the deal in place would have led to a nuclear arms race in the Middle East, he said.

It was a "horrible one-sided deal that should never ever have been made". He added that the United States "will be instituting the highest level of economic sanction".

US President Donald Trump shows a signed Presidential Memorandum after delivering a statement on the Iran nuclear deal from the Diplomatic Reception Room of the White House. Photo / AP
US President Donald Trump shows a signed Presidential Memorandum after delivering a statement on the Iran nuclear deal from the Diplomatic Reception Room of the White House. Photo / AP

Trump's decision means Iran's government must now decide whether to follow the US and withdraw or try to salvage what is 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.

One official briefed on the decision said Trump would move to reimpose all sanctions on Iran that had been lifted under the 2015 deal, not just the ones facing an immediate deadline.

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

The Iran agreement, struck in 2015 by the United States, other world powers and Iran, lifted most US and international sanctions against the country. In return, Iran agreed to restrictions on its nuclear programme making it impossible to produce a bomb, along with rigorous inspections.