Top-level talks began in the Swiss mountain resort of Bürgenstock on Sunday between US Vice President JD Vance and Iranian representatives as they seek to hammer out a long-term peace deal after signing a memorandum of understanding earlier this week.
The Foreign Ministry of Qatar, which is mediating between the warring countries alongside Pakistan, announced the start of the negotiations.
A spokesman for the ministry said that an initial high-level meeting involving representatives from the US and Iran as well as delegations from Pakistan and Qatar was under way.
The talks were overshadowed by reports of further clashes between Israeli forces and the Hezbollah militia in Lebanon despite the ceasefire in the conflict.
Iran's Foreign Ministry played down any high expectations from the talks. Speaking to state broadcaster IRIB, ministry spokesman Ismail Baghai said there were no plans to extend them beyond a single day.
Baghai said the talks would start with bilateral meetings between the Iranian representatives and the Pakistani and Qatari delegations followed by a round with the US delegation in the afternoon.
Working groups at expert level were also planned, he said. These would deal with the release of blocked Iranian bank accounts and the possible easing of sanctions, he added.
It was not clear whether these talks would also be terminated after a single day or were being conducted independently of the political negotiations. Nasser Hemmati, governor of Iran's central bank, has also travelled to Switzerland.
Under the terms of the framework agreement between Washington and Tehran, a final agreement is to be reached within 60 days on Iran's nuclear programme.
However, the truce already appears to be vulnerable, with Iran's military announcing on Saturday it was again closing the Strait of Hormuz to all shipping in response to the ceasefire violations in Lebanon.
Trump meanwhile threatened to impose a US-controlled transit fee in the key waterway if a deal is not achieved within the 60-day timeframe.
Ahead of his departure, Vance told reporters: "I think we're going to hopefully make progress on the nuclear issue, make progress on the Lebanon ceasefire issue."
He added that US special envoy Steve Witkoff and Trump's son-in-law Jared Kushner were already in Switzerland and that he expected to spend at most two days in the country.
The Iranian delegation is expected to include Parliament Speaker Mohammad Bagher Ghalibaf and Foreign Minister Abbas Araghchi. Pakistani Prime Minister Shehbaz Sharif is also to be present.
An initial round of talks was scheduled for Friday but was called off after renewed clashes in Lebanon.
