US President Donald Trump and Iranian President Hassan Rouhani have traded hostile warnings, amid rising tensions between the two countries.
Mr Trump tweeted that Iran "will suffer consequences the likes of which few throughout history have ever suffered before" if it threatened the US.
Mr Rouhani earlier said that war with Iran would be "the mother of all wars".
In May, the US left a deal which curbed Iran's nuclear activities in return for the lifting of international sanctions.
Washington is now re-imposing the sanctions, despite objections from the UK, France, China, Russia and Germany, who all signed the 2015 agreement.
President Rouhani's comments, made to Iranian diplomats, did leave open the possibility of future good relations with the US.
"America should know that peace with Iran is the mother of all peace, and war with Iran is the mother of all wars," he said, according to Iran's state news agency Irna.
Mr Trump's angry rhetoric has echoes of his Twitter barrages against North Korea's Kim Jong-un, whom he branded a "madman" who "will be tested like never before", before engaging in a testy exchange over whose nuclear button was bigger.
Their verbal hostilities nonetheless evolved into ongoing diplomatic negotiations.
'More of a mafia than a government'
In a separate development on Sunday, US Secretary of State Mike Pompeo said the Iranian regime "resembles the mafia more than a government".
Addressing a group of Iranian Americans in California, Mr Pompeo denounced Mr Rouhani and Iranian Foreign Minister Javad Zarif, who negotiated the nuclear deal, as "merely polished front men for the ayatollahs' international con artistry".
Mr Pompeo said he wanted to try to stop countries importing Iranian oil by November as part of continued pressure on Tehran.
The gathering was the first time a top US official had directly addressed such a large number of Iranian Americans, says BBC state department correspondent Barbara Plett Usher. It is being seen as part of the administration's strategy to increase pressure on Iran's leadership.