A rare visit by senior UN nuclear inspectors next week raises pressure on Iran to address suspicions it is trying to develop atomic weapons, though Western powers that are piling on sanctions expect no significant breakthrough.
How Tehran deals with the International Atomic Energy Agency may offer pointers to prospects for resolving a long-running dispute that an oil embargo and threats of war, along with talk of Iran closing in on nuclear weapons capability and the US presidential election calendar, have escalated to crisis level.
The outcome of the inspectors' meetings from Sunday to Tuesday will be closely watched in Washington, European capitals and Israel for signs of whether Iran's leadership may finally be prepared to give ground after a decade of pursuing its nuclear development goals or whether it remains as defiant as ever.
"I hope that, at a minimum, the parties can agree on how to proceed in resolving outstanding issues, and that also includes the military dimension of Iran's nuclear programme," said Olli Heinonen, a former chief UN safeguards inspector.
Western diplomats, who have often accused Tehran of using offers of meetings as a stalling tactic while pressing ahead with its nuclear programme, say they doubt that it will lead to the kind of concrete progress the UN agency is looking for.
But some speculate that Iranian officials could show more openness or make limited concessions in the discussions in Tehran with senior IAEA delegates in a bid to keep channels of dialogue open and avert even more punitive steps.
"We are sceptical that anything substantive is going to come out of the meeting," one Vienna-based diplomat said.
But, "it would be a smart move on their part to do something, to provide some sort of...cooperation."
The IAEA team, to be headed by global inspections chief Herman Nackaerts, is expected to call for access to sites, officials and documents that could help clarify mounting concerns that Iran may be trying to develop nuclear arms.
The IAEA issued a detailed report in November that laid bare a trove of intelligence suggesting Iran is seeking nuclear weapons capability, including accusations of work on bomb triggers and computer-simulated detonations.
Washington and its European allies, vowing not to tolerate any Iranian push for nuclear bombs, seized on the IAEA document to ratchet up sharply the sanctions pressure on Iran. Financial measures and bans on buying Iranian oil aim to choke its budget.
Tehran, which some experts say could have the potential to build at least one nuclear device as early as next year, responded by threatening to disrupt global oil trade.
There is speculation that Israel, which says an Iranian atom bomb would threaten the survival of the Jewish state, might launch attacks on the country's nuclear sites. US President Barack Obama also says force is an option. He faces criticism in his re-election campaign this year of being "soft" on Tehran.
Iran, which says its nuclear programme is entirely peaceful, angrily dismissed the IAEA findings as fabricated and baseless.
But it has recently sent more conciliatory signals to the agency in Vienna, inviting Nackaerts and his aides to visit and saying it is ready to discuss "any issues" that interest them.
"Iran will have to at least give the appearance of having dealt substantively with the issue," one European diplomat said, but added: "I would be shocked if Iran actually confessed to anything."
Making clear he did not expect quick results, Heinonen said the IAEA mission could be "the first step in a long process where the first real results would emerge in coming months".
Ali Vaez, an expert at the Washington-based Federation of American Scientists, said Iran's goal was to avoid being referred once again to the U.N. Security Council by the IAEA's 35-nation governing board, which meets in early March.
The IAEA board groups both Western states pushing to isolate Iran, and Russia and China which are critical of unilateral measures. It first reported Iran in 2006 to the Security Council, which has since imposed four rounds of sanctions on the country, a major oil producer.
"Sporadic cooperation with the UN nuclear watchdog is a consistent feature of Iran's dual-track strategy of dividing the international community and deflecting additional sanctions," Vaez said.
In line with this, Iran has indicated readiness to address the concerns of the IAEA - tasked with preventing the spread of nuclear arms - about possible military dimensions to its nuclear programme in the meeting with the visiting agency officials.
"Iran has in the past continued to question the IAEA mandate on these topics. I hope that we can now put that era behind us," said Heinonen, Nackaerts' predecessor at the IAEA and now a senior fellow at Harvard University.
Separately, but closely linked to the IAEA's discussions, Iran has also voiced willingness to resume wider nuclear negotiations with world powers that have been frozen for a year.
"Iran is prepared to engage in dialogue ... over the nuclear issue on the basis of mutual respect," chief Iranian nuclear negotiator Saeed Jalili was quoted as saying in Iranian media.
But Western officials say Iran has yet to back this up with a specific offer of meaningful talks about its uranium enrichment programme, which Tehran is refusing to suspend despite repeated UN resolutions calling on it to do so.
Iran has in previous meetings spurned offers from the six powers - the United States, China, Russia, Britain, France and Germany - of economic and other incentives if it curbs such work, which can have both military and civilian purposes.
Tehran says it is enriching uranium to fuel planned nuclear power plants and sees it as a source of prestige in testament to its technical and scientific prowess. But the material can also be used for atomic bombs if it is processed much further.
Last week, diplomats said the six powers were split over what to put on the table should talks resume and on whether to allow Iran to continue enrichment to some degree.
Echoing the views of other Western experts, a former British ambassador to the IAEA advocated a deal under which Iran could keep on refining uranium if it accepted stricter UN safeguards inspections to ensure it had intention to build nuclear weapons.
However, Peter Jenkins wrote in the Daily Telegraph, "it may be asking a lot of our leaders that they swallow their words, lower their sights and focus on a realistic target"