"I, as of now, announce the third step," he said on state television. "The atomic energy organisation (of Iran) is ordered to immediately start whatever is needed in the field of research and development, and abandon all the commitments that were in place regarding research and development."

Iran in July abandoned two other nuclear commitments: to keep its stockpile of enriched uranium below 300-kilogrammes, and a 3.67-percent cap on the purity of its uranium stocks.

Rouhani had told his cabinet earlier Wednesday that Iran would likely announce a new step in scaling back its nuclear commitments within a day as it seeks sanctions relief from the United States.

Iran and European countries have been engaged in talks to strike a deal to reduce tensions and save the 2015 deal that has been unravelling since US President Donald Trump withdrew from it in May last year.

But Rouhani told ministers: "I don't think that... we will reach a deal so we'll take the third step and we will announce the details today or tomorrow."