Geneva, Switzerland - Iran's president said the Islamic republic was not "at all" seeking nuclear weapons as talks with the US were due to begin in Switzerland on Thursday in a last-ditch bid to stop President Donald Trump from launching an unprovoked war.

The talks mediated by Oman follow a massive US military build-up in the region not seen in decades, Trump threatening to strike Iran if his demands are not met.

"Our Supreme Leader has already stated that we will not have nuclear weapons at all," President Masoud Pezeshkian said, in a reference to Ayatollah Ali Khamenei.

While Iran has insisted the talks focus solely on its nuclear program, the US – egged on by Israel – wants Tehran's missile arsenal and support for militant groups in the region massively reduced.

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In his State of the Union address on Tuesday, Trump accused Iran of "pursuing sinister nuclear ambitions," even though the country has always insisted its program is for civilian purposes.

Trump also baselessly claimed Iran had "already developed missiles that can threaten Europe and our bases overseas, and they're working to build missiles that will soon reach the United States of America".

The Iranian foreign ministry called these allegations, repeated by Secretary of State Marco Rubio on Wednesday, "big lies."

The maximum range of Iran's missiles is 1,200 miles, according to what Tehran has publicly disclosed. The Congressional Research Service estimates they top out at close to 1,900 miles – less than a third of the distance to the continental US.

Foreign Minister Abbas Araghchi, who is leading the Iranian delegation at the talks, has called them "a historic opportunity", adding that a deal was "within reach".

In a foreign ministry statement that followed a meeting with his Oman counterpart, Araghchi said the success of the US negotiations depend "on the seriousness of the other side and its avoidance of contradictory behavioR and positions".

The US will be represented by envoy Steve Witkoff and Trump's son in law, Jared Kushner.

A previous attempt at negotiations collapsed when Israel launched unilateral strikes on Iran last June, beginning a 12-day war that the US briefly joined to bomb Iranian nuclear sites.