President Trump has condemned Iran as a “fanatical regime” and refused to continue certifying an international nuclear deal.
He said he was referring the deal to Congress and would consult allies on how to alter the deal.
He accused Iran of sponsoring terrorism and said he would deny the regime “all paths to a nuclear weapon”.
International observers say Iran has been in full compliance with the 2015 deal freezing its nuclear programme.
But Mr Trump said the deal was too lenient, and Iran had been allowed to exceed heavy-water limits and intimidate international inspectors.
Iran, he said, was “not living up to spirit of the deal” but was receiving the benefit of sanctions relief regardless.
He said that US reserved the right to leave the deal at any time.