Iran has "sanitized" to such an extent a military base where nuclear weapons research allegedly took place that the UN atomic watchdog may say next week there is now little point inspecting it, Western diplomats told AFP.
The International Atomic Energy Agency has been pushing Iran to allow access to Parchin, most recently at a failed meeting in Vienna on Friday, where it suspects explosives testing consistent with nuclear bomb research occurred.
Iran, subject to unprecedented Western sanctions and amid heightened speculation of Israeli military action, denies seeking or ever having sought nuclear weapons but has so far blocked the IAEA's requests to see the site.
Western nations have accused Iran of bulldozing parts of the sprawling base near Tehran and the IAEA said in May that activities spotted there by satellite "could hamper the agency's ability to undertake effective verification."
On August 1 US think-tank the Institute for Science and International Security (ISIS) published open-source satellite images showing "what appears to be the final result of considerable sanitization and earth displacement activity."
New ISIS images Friday on its website (http://isis-online.org/) showed a building suspected of housing the explosive experiments covered in pink tarpaulin in what Western diplomats said was an attempt to hide activity from satellites.
Ali Asghar Soltanieh, Iran's ambassador to the IAEA, has called accusations of a clean-up at Parchin "a childish, ridiculous story made out of nothing."
One Western diplomat told AFP on condition of anonymity on Friday that the IAEA is so frustrated that in its next quarterly report on Iran, expected next week, it may say that going to Parchin now would serve little purpose.
"I would expect language in the report saying 'you are clearly sanitizing, the satellite imagery shows that, and frankly once you let us in, you have done so much it is going to be irrelevant, academic'," the envoy said.
A second Western diplomat told AFP on Saturday that the IAEA saying something along those lines "is certainly something that would make sense, although we don't know definitively how they are going to characterise it in their report."
"We think any value of a visit to Parchin now is greatly diminished," the envoy said on condition of anonymity.
The first diplomat also said that as a result, and after Friday's fruitless meeting, Western nations might table a resolution sharply criticising Tehran at the next IAEA board of governors' meeting starting September 10.
"We are getting nowhere swiftly ... We need to make a more formal and public example of the failure of the sides to engage, which is Iran's fault," he said, adding however it was unclear whether Russia and China would support such a move.
IAEA inspectors visited Parchin twice in 2005 but want to look at it again after new information came to light.
Although analysts say other sites are more significant, the IAEA has zeroed in on Parchin because its information on the site, unlike on others, is its own and not from foreign intelligence services.
Iran has said it will allow monitors access only as part of a wider arrangement governing relations between Iran and the watchdog, which experts and diplomats say would limit to an unacceptable degree the IAEA's inspection rights.
The IAEA report next week is also expected to say that despite the pressure, Tehran is continuing to expand its programme by installing several hundred new centrifuges in its Fordo plant, dug into a mountain so difficult to bomb.