Iranian President Hassan Rouhani has warned that the US will face “historic regret” if Donald Trump scraps the nuclear agreement with Tehran.
Mr Rouhani’s comments come as the US president decides whether to pull out of the deal by a 12 May deadline.
Mr Trump has strongly criticised the agreement, calling it “insane”.
The 2015 deal – between Iran, the US, China, Russia, Germany, France and the UK – lifted sanctions on Iran in return for curbs on its nuclear programme.
France, the UK and Germany have been trying persuade the US president that the current deal is the best way to stop Iran developing nuclear weapons.
British Foreign Secretary Boris Johnson is travelling to Washington on Sunday to discuss the matter with White House officials.
The UN also warned Mr Trump not to walk away from the deal.
However, he has threatened that the US will “withdraw” from the deal on 12 May – the end of a 120-day review period – unless Congress and European powers fixed its “disastrous flaws”.
In remarks carried live on Iranian state television on Sunday, President Rouhani said: “If America leaves the nuclear deal, this will entail historic regret for it.”
He warned Iran had “a plan to counter any decision Trump may take and we will confront it”.
Iran insists its nuclear programme is entirely peaceful and says it considers the deal non-renegotiable.