The UN nuclear watchdog hopes to persuade Iran in talks on Saturday to finally start addressing long-held suspicions it has worked on designing an atomic bomb, a test of whether ties really are thawing under the Islamic state's new president.
The International Atomic Energy Agency has made it clear it now wants Iran to end what the West sees as years of stonewalling the IAEA's investigation into alleged nuclear bomb research by the country, which denies any such activity.
Diplomats are cautiously optimistic that the team of senior IAEA inspectors will return from the meeting in Tehran - which may run into Sunday - able to show at least some progress in gaining Iran's cooperation with the investigation.
However, they said the IAEA may tread carefully to avoid upsetting the delicate building of rapport at a time when Iran and six big powers are due to start separate, high-stakes talks on a broader settlement of the decade-old nuclear dispute.
The UN agency may therefore try to begin with getting Iran to clarify questions about some of the less sensitive aspects of the IAEA's inquiry into what it calls the "possible military dimensions" (PMD) to Tehran's nuclear program.
"I think they absolutely have to start with some PMD issues. Low-hanging fruit would be fine as long as it was real PMD," a Western diplomat said.
The investigation "is about being thorough and transparent, not about being fast," the envoy, who closely follows Iran's nuclear program but is not from any of the big powers, said.
This probably means that the IAEA's long-sought access to the Parchin military base, where it believes explosives tests relevant for nuclear bombs took place a decade ago, may have to wait a while longer.
The IAEA wants Iran to clarify suspected activities in a range of areas of potential application to developing atomic bombs, including computer calculations and experiments that could be of use for any nuclear test.
Tehran has rejected the accusations of westernization-related work as forged and baseless, while saying it will cooperate with the IAEA to clear up any "ambiguities".
The February 8 meeting comes 10 days before Iran and the world powers, building on a landmark interim deal struck in November, start negotiations on a long-term agreement on Tehran's nuclear aspirations that would avert the threat of a Middle East war.
IAEA SEEKS "SUBSTANTIVE MEASURES"
Western diplomats hope those talks will lead to Iran scaling back its nuclear program sufficiently to deny it the capability to assemble a nuclear weapon any time soon.
The IAEA's investigation is focused on the question of whether Iran sought atomic bomb technology in the past and, if it did, to determine whether such work has since stopped.
Although separate, it is still closely aligned with the wider-ranging diplomacy between Tehran and the six powers - United States, Russia, China, France, Britain and Germany.
Western officials stress that progress in the U.N. inquiry will be important to reaching a diplomatic accord, even though some analysts question how hard the powers in the end will press for IAEA closure as part of any final deal.
"A satisfactory resolution of these issues will be crucial to any comprehensive solution," the US ambassador to the IAEA, Joseph Macmanus, said last month.
Iran says its nuclear program is peaceful and denies Western and Israeli accusations that it is seeking the means and expertise to assemble a nuclear warhead. It says it is Israel's assumed atomic arsenal that threatens peace.
Often strained in the past, relations between Iran and the IAEA have improved since last year's election of a relative moderate, Hassan Rouhani, as Iranian president on a platform to ease Tehran's international isolation.
Under an agreement signed just weeks before the powers reached their own initial accord in late November to cap Iran's nuclear activity, the IAEA has already visited a heavy water production plant and a uranium mine in Iran.
However, those first steps did not go to the heart of its investigation and Western diplomats will watch Saturday's meeting closely to see whether and to what extent the next phase achieves that.
IAEA Director General Yukiya Amano said this week he hoped the talks in Tehran would lead to an agreement on "substantive measures", without elaborating.