Talks between Iran and the US aimed at building out the fragile interim deal to end the war are due to get under way in Switzerland, beset by difficulties including an Iranian decision to keep the strait of Hormuz closed in protest at Donald Trump’s inability to force Israel to end the fighting in Lebanon.

The US vice-president, JD Vance, leading the US delegation, said he was adding Lebanon to the agenda, which had originally been conceived to focus on the opening of the strait, the lifting of US sanctions on Iranian oil exports and the unfreezing of Iranian assets held overseas.

Vance arrived at a Qatari-owned Swiss mountainside resort in Bürgenstock early on Sunday to meet Iranian negotiators for the second time since the months-long conflict began. He has already held direct talks with the Iranians in Islamabad.

The Iranian delegation is led by the speaker of the parliament, Mohammad Bagher Ghalibaf, but the presence of the deputy oil minister and the governor of Iran’s central bank shows how Iran had wanted to focus on the terms for lifting sanctions.

The first clause of the memorandum of understanding published last week requires a ceasefire on all fronts including Lebanon, where fighting has escalated between Israel and the Iranian-backed militant group Hezbollah.

Vance said: “I think we’re going to hopefully make progress on the nuclear issue, make progress on the Lebanon ceasefire issue. Those are the two big things that I think we’re to be focused on.” He said he could join the talks only “for a day or two”.

The Swiss foreign ministry said the US and Iranian delegations, plus mediators from Pakistan and Qatar, were all present at the luxury resort, with talks set to begin later on Sunday morning.

The practical impact of the Iranian decision to keep the strait of Hormuz closed is yet to be tested, but Trump said last week that the world was four weeks from running out of sufficient refined oil, and said there would have been worldwide recession if he had not agreed to the strait’s reopening by lifting the US blockade on Iranian oil ports.

He is under fierce attack from US supporters of Israel who are angry at the personal attacks mounted on the Israeli prime minster, Benjamin Netanyahu, by Trump and Vance.

The Iranian delegation arrived in Switzerland with a furious row exploding in Tehran between advocates of negotiations and those that insist pursuing an agreement with the US is purposeless because the US cannot be trusted.

Mahmoud Nabavian, a long-term critic of the negotiations and a member of the delegation that went to Islamabad, claimed the negotiations being pursued were “fundamentally different” to the conditions originally approved by the new supreme leader, Mojtaba Khamenei. Last week, Khamenei published a letter saying he opposed the talks but in deference to the president, Masoud Pezeshkian, had permitted the talks to proceed so long as the interests of the axis of resistance – a reference to Hezbollah – were protected.

Nabavian’s appearance on state TV was cut short and it was announced legal action would be taken against him. A senior official from the state broadcaster Irib was reportedly to resign.

During his appearance, Nabavian claimed correspondence had been sent by the supreme leader three times limiting the scope of the proposed talks. He claimed that in one letter referring to the Islamabad talks, Khamenei wrote: “What was achieved in the negotiations is completely different from what was supposed to be done and was a condition for the legitimacy of the negotiations.”

Referring to the management of the strait of Hormuz, Nabavian said: “The leader of the revolution has emphasised Iran’s monopoly on the management of the strait of Hormuz, collecting tolls from passing vessels, restrictions on enemy ships, and allocating the revenues from the tolls to the people, families of martyrs, and veterans.”

The claim that Iran’s negotiators may be ignoring the guidelines set by the supreme leader would be regarded as a very serious charge inside Iran.

Pezeshkian tried to calm the mood, saying: “All the provisions of the memorandum of understanding signed between Iran and the United States are in our favour, and the achievements of this dialogue and negotiation will be made clear.”

Pezeshkian said the words of the US president had changed 180 degrees. “Trump, who forbade us from doing many things, in his recent speech considered all of them the right of the people and the nation. Our $6bn in money will be returned from Qatar.”

Pointing out that Netanyahu was the first to be dissatisfied with the negotiations, he added: “The achievements of this dialogue and negotiation will be revealed. All the provisions of the memorandum are in our favour.”

The only American demand “is that we not have atomic bombs; this is something that the martyred leader has said many times: ‘We do not want atomic bombs.’ The United States said ‘write this and sign it’ and we signed.”

There remained scepticism from Tehran, however, with Mohsen Rezaei, a military adviser to Khamenei, posting on X: “The enemy has shown itself to be a promise-breaker.” He said: “One should be cautious; any optimism will be exploited by the enemy.”

Hojjatoleslam Sadeghi, the supreme leader’s representative to the Islamic Revolutionary Guard Corps, said: “Our enmity with arrogance will never end. We are not negotiating to reconcile, but rather to get our rights.”

Washington announced a renewed ceasefire in Lebanon later on Friday, but Israeli troops again clashed with Hezbollah fighters the following day, with each side accusing the other of breaking the truce. Citing a US “breach of contract”, Iran’s central military command said it was closing the strait of Hormuz to vessel traffic.