Amir Saeid Iravani, the Iranian ambassador to the UN, made the remarks in a letter to UN secretary-general Antonio Guterres and the president of the Security Council that was seen by AFP.
President Donald Trump has deployed warships, fighter jets and other military hardware to the Middle East as he seeks to block Iran from building a nuclear bomb, something Tehran says it is not pursuing.
The letter cited a social media post by Trump on Wednesday, where he said the US may need to use UK military bases, including one on an Indian Ocean island, “should Iran decide not to make a deal”.
“Such a belligerent statement by the President of the United States…signals a real risk of military aggression, the consequences of which would be catastrophic for the region and would constitute a grave threat to international peace and security,” Iravani wrote in the letter.
He called for the Security Council – the UN’s top decision-making body where Washington has veto power – to “ensure that the US immediately ceases its unlawful threats of the use of force.”
The letter said Iran remains committed “to diplomatic solutions” and, “on a reciprocal basis, addressing ambiguities regarding its peaceful nuclear programme.”
But Iravani warned if Iran faced military aggression, “all bases, facilities, and assets of the hostile force in the region would constitute legitimate targets in the context of Iran’s defensive response.”
Trump has repeatedly threatened military action against Iran, citing a deadly crackdown on anti-government protesters last month and more recently over its nuclear programme.
On Thursday, Trump said that Iran had at most 15 days to make a deal and again suggested that the US would attack if it failed to do so.
His comments followed talks on Tuesday in Geneva between US envoys Steve Witkoff and the president’s son-in-law Jared Kushner, who met indirectly with Iran’s top diplomat, who said that there was progress.
A previous attempt at negotiations collapsed when Israel launched surprise strikes on Iran last June, beginning a 12-day war that Washington briefly joined to bomb Iranian nuclear sites.