The UN atomic agency's dogged focus on the Parchin military site in its probe into Iran's suspected nuclear weapons work is putting Tehran in a tight spot ahead of hugely important talks with world powers.
In two high-level visits to Tehran this year, in January and last month, the International Atomic Energy Agency asked to go to Parchin, where it believes activity relevant to nuclear weapons development took place.
But Iran said no, making instead a last-minute offer to show another site mentioned in a major IAEA report in November -- Merivan near the Iraqi border, hundreds of miles from Tehran -- which the IAEA refused.
Inspectors already visited Parchin near Tehran twice in 2005 and found nothing, Iran points out, but the IAEA says it has since obtained additional information that makes it want to go back for another look.
For Mark Fitzpatrick, nuclear nonproliferation expert at the International Institute for Strategic Studies in London, Iran's rebuttal of the IAEA was an "own goal", particularly as there is likely nothing "incriminating" at Parchin.
The studies that Iran is alleged to have carried out there, although believed to be aimed at developing nuclear weapons, did not use any radioactive material, making detecting something much harder for the IAEA, Fitzpatrick told AFP.
Refusing access "just raises suspicions. Iran would have been much more clever to have brought them to Parchin ... It would have been a PR victory for Iran and they blew their chance," he said.
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Iran is hyper-sensitive about allowing access into military sites, particularly since a November blast at an elite Republican Guard base killed 36, including a key figure in Iran's ballistic missile programme.
It has already accused the IAEA of being dangerously prone to leaks and of endangering the lives of its nuclear scientists -- several have been assassinated, the latest in January -- by making their names public.
"Considering the fact that it is a military site, granting access is a time-consuming process and cannot be permitted repeatedly," the Iranian embassy in Vienna, where the IAEA has its headquarters, said in a statement this week.
But the IAEA thinks there is more to the refusal than security concerns, while Iran's stance falls into the hands of those -- not least Israel -- who suspect that Tehran is secretly bent on developing a nuclear arsenal.
Access might be one concession Iran could make in upcoming talks with the United States, China, Russia, Britain, France and Germany, said Oliver Thraenert from the German Institute for International and Security Affairs (SWP) in Berlin.
"If the Iranians are clever they would give access to Parchin, but try at the same time to organise it in a way that the inspectors can have general access but not access to every single facility," Thraenert told AFP. "It's a huge place."
But if Tehran does suddenly grant access to Parchin, it may be too late because it will find itself accused of having cleaned up the site beforehand.
The director general of the IAEA, Yukiya Amano, strongly hinted as much on Monday, saying that "activities" spotted by satellite "makes us believe that going there sooner is better than later."