Washington DC - President Donald Trump said he has written to Iran pressing it to open talks on preventing the development of nuclear weapons or face possible military action.

"I've written them a letter, saying I hope you're going to negotiate because if we have to go in militarily it's going to be a terrible thing for them," Trump told Fox Business in a clip broadcast Friday.

"You can't let them have a nuclear weapon."

A landmark 2015 deal – known as the Joint Comprehensive Plan of Action (JCPOA) – imposed curbs on Iran's nuclear program in return for sanctions relief.

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It fell apart after Trump withdrew the US from the agreement in 2018 in his first presidential term.

Tehran adhered to the deal until a year after Washington pulled out unilaterally, but then began rolling back its commitments.

Trump, who returned to the White House in January, reinstated his "maximum pressure" policy of sanctions against Iran over allegations the country is seeking nuclear weapons capability.

Tehran has consistently denied the claims and repeatedly expressed a willingness to revive the accord, but has been met with little encouragement by the US and its Western allies.

Last month, UN atomic agency chief Rafael Grossi said Iran was "enriching (uranium) at 60%, so almost weapon level" and that the 2015 deal was an "empty shell" that was "no longer fit for purpose."