Iran and the world’s big powers are set to begin their first negotiations on Tehran’s nuclear programme in more than a year in talks that could determine the likelihood of a military strike or the possibility of a diplomatic breakthrough.

As officials arrived in Istanbul on Friday, western diplomats discounted the prospect of a quick deal, suggesting instead that the key task was to avoid a complete breakdown in negotiations.

The last such meeting, in January last year, failed to make any progress - an outcome other countries blamed on Iran’s refusal to discuss its nuclear programme without preconditions. But this time the stakes are much higher, largely because of signals that Israel could launch a military strike against the programme unless it is soon reined in.

“For us the most important thing is that the meeting is taking place,” said an official from Turkey, which is hosting the talks but not directly taking part itself. “The decision to continue talks will be a success in itself.”

Other diplomats say the key challenge is to ensure the talks are substantive enough to continue the process – which means not only addressing the nuclear programme overall, but also particular areas of international concern. These include Iran’s production of enriched uranium at a relatively high level of purity and operations at a previously clandestine mountain facility at Fordow, near the holy city of Qom.

While Iran insists its programme is wholly peaceful, the US and its allies suspect it is seeking the capacity to build a nuclear weapon.

“The question for the first encounter is whether the Iranians turn up to dance,” said a European diplomat. “The question over the next period will be how serious the overtures are, the extent to which we have a productive set of conversations or just talks for talks’ sake.”

The meetings were set to begin with a dinner between Sayeed Jalili, the top Iranian negotiator, and Catherine Ashton, European Union foreign policy chief, who leads the international delegation of US, Russian, Chinese, British, French and German diplomats. Formal negotiations will begin on Saturday.

But one potential issue facing the world’s big powers is the uneasy nature of their coalition. Although all have called on Iran to cease uranium enrichment, which can produce both nuclear fuel and weapons-grade material, they have not yet submitted compromise proposals to Iran, arguing that it is important to maintain flexibility for the negotiations themselves.

This week Sergei Ryabkov, Russian deputy foreign minister, said the big powers – known as the E3 plus 3 – “really do not have a common view of what’s the real offer to be made to Iran to bring it to serious negotiations”, although other diplomats insist they are united.

Barack Obama, US president, may also be constrained in negotiations by election year Republican attacks on his alleged policy of “appeasement”, particularly after North Korea carried out an alleged abortive missile test on Friday just weeks after agreeing a tentative deal with Washington.

In a sign that Iran was approaching the talks in a less intransigent mood than in the past, Ali Salehi, Iran’s foreign minister, wrote in the Washington Post that the talks had to re-establish confidence between Tehran and the west.

“In the upcoming talks,” he wrote, ”we hope that all sides will return to the negotiating table as equals with mutual respect; that all sides will be committed to comprehensive, long-term dialogue aimed at resolving all parties’ outstanding concerns; and, most important, that all sides make genuine efforts to re-establish confidence and trust.”

He also noted a fatwa issued by Ayatollah Ali Khamenei, Iran’s supreme leader, forbidding the production, stockpiling and use of nuclear weapons.

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