"The way is open for indirect negotiations," Foreign Minister Abbas Araghchi said, dismissing the prospect of direct talks "until there is a change in the other side's approach towards the Islamic republic".

The top Iranian diplomat said Tehran would not engage in direct talks with Washington under threats and so long as Trump maintain his "maximum pressure" policy.

Under that policy in his first term as president, Trump withdrew the United States from a landmark agreement on Iran's nuclear programme in 2018 and reimposed crippling sanctions on Tehran.

The deal, sealed in 2015 between Tehran and Western powers, required Iran to limit its nuclear ambitions in exchange for sanctions relief.

Western countries including the United States have long accused Iran of pursing a nuclear weapon, which Tehran has denied, insisting its enrichment activities were solely for peaceful purposes.

On March 7, Trump said he had written to Iran's supreme leader Ayatollah Ali Khamenei to call for nuclear negotiations and warn of possible military action if Tehran refused.

The letter was delivered to Tehran on March 12 by UAE presidential adviser Anwar Gargash, Iranian news agency Fars reported at the time.

On Friday Khamenei said US threats "will get them nowhere", warning of reciprocal measures "if they do anything malign" against Iran.

Araghchi on Thursday said Trump's letter was "more of a threat", but added that it could also open up some opportunities and that Tehran would respond soon.

US Middle East envoy Steven Witkoff, in an interview published Friday, said Trump's goal was to avoid military conflict by building trust with Iran.

He insisted the letter was not meant as a threat.

Tehran and Washington cut diplomatic ties after Iran's 1979 Islamic Revolution that toppled the Western-backed shah.

Since then, the Swiss embassy in Tehran has facilitated communications between the two nations.

Gulf state Oman has also mediated indirect talks on Iran's nuclear issue via the so-called "Muscat process", which Araghchi had said in October was "halted for the time being."