Iran has said it would not allow any of its enriched uranium stockpile to be shipped abroad, but could sanction other curbs on its nuclear programme to reassure the international community, it is not interested in building a bomb.

The statement, the clearest outline of Iran's negotiating position since the election in June of President Hassan Rouhani, was delivered as an Iranian negotiating team prepared to fly out for nuclear talks in Geneva on Tuesday, with hopes of a breakthrough at their highest point in four years. A senior member of the team, the deputy foreign minister, Abbas Araqchi, told Iranian state television: "Of course we will negotiate regarding the form, amount, and various levels of [uranium] enrichment, but the shipping of materials out of the country is our red line."

In earlier rounds of nuclear negotiations, a group of six major powers had suggested a confidence-building measure by which Iran would stop producing 20%-enriched uranium – the main proliferation concern – ship out its stockpile and shut down its underground enrichment plant at Fordow, in exchange for limited sanctions relief, on trade in gold, precious metals and petrochemicals.

The 20% enriched uranium is seen as a particular worry because it could relatively easily and quickly be turned into weapons-grade uranium (90% enriched and above – the percentages refer to the concentration of the highly fissile isotope, U-235). Israel's prime minister, Binyamin Netanyahu has said that Iran's accumulation of enough 20% uranium to make a bomb – about 250kg – would trigger a military response. The Iranian stockpile is currently about 190kg.

However, a refusal to ship this medium-enriched uranium out of the country will not necessarily be a deal-breaker in Geneva. In previous discussions, the option has been floated of keeping it under surveillance by the International Atomic Energy Agency (IAEA) in a remote part of Iran such as the island of Kish. Furthermore, the clear statement from Araqchi that his country was willing to accept curbs on its future enrichment activities will be seen as encouraging by the diplomats arriving in Switzerland from the six-nation group: the US, UK, France, Germany, Russia and China. The group is chaired by the EU foreign policy chief, Catherine Ashton.

After positive talks at the UN general assembly last month, Ashton asked Iran to send its proposals early so a response could be prepared before this week's two-day talks. But the Iranian foreign minister, Mohammad Javad Zarif, who will lead the delegation to Geneva, said on his English-language Twitter account on Friday: "We will present our views, as agreed, in Geneva, not before. No Rush, No Speculations Please (of course if you can help it!!!)"

Four years ago in Geneva, Iran struck a tentative agreement to stop 20% enrichment, but the deal was later rejected by conservatives in Tehran and ultimately by the Supreme Leader, Ali Khamenei.

However, an Iranian official said President Rouhani was in a much stronger political position in Tehran than his predecessor, Mahmoud Ahmadinejad, who was locked in a long-running power struggle with Khamenei.

"Rouhani has the full backing of the Supreme Leader, but not only that. He is respected as a veteran of the [1980s] war with Iraq, by the military and the Revolutionary Guards," the official said. "People feel that Rouhani has always been with us. With Ahmadinejad they didn't feel that way. They didn't know where he had come from."

The official said that the Rouhani government was confident a deal could be done. When asked whether Iran would be prepared to accept strict curbs on its enrichment programme and more stringent IAEA inspections in return for sanctions relief and the recognition of Iran's right to enrich uranium in principle, the Iranian official nodded. But he added: "Everything depends on how this is done, the sequencing. Rouhani and Zarif has to be able to show he is gaining something for Iran - that this is not a trick."

William Luers, a veteran US diplomat who runs the Iran Project advocating reconciliation with Tehran, said: "I get the impression from talking to both Zarif and Rouhani … that they have made a decision that they want to open up their economy to the world again, and are prepared to do substantial things to make that happen. And they will say: 'We want to know what you will do in terms of sanctions relief.'"If progress is made at the talks, the impasse between Congress and President Barack Obama could become a significant obstacle as most US sanctions have been imposed by Congress and would require congressional approval to be limited permanently. However, European officials point out that Obama has the power to suspend US sanctions and that the EU could independently lift its oil embargo and financial sanctions if it was thought necessary to maintain momentum towards a comprehensive deal.