The UN atomic agency's chief inspector said Wednesday that it may now not find any evidence of nuclear weapons research at Iran's Parchin base because of a clean-up by Tehran, according to diplomats.

The comments from Herman Nackaerts came as he showed member states of the International Atomic Energy Agency new satellite imagery of the military site near Tehran at a closed-door briefing in Vienna, the envoys said.

Nackaerts "said that they can find evidence of nuclear material but he also said that it was possible they might not because of the clean-up," one diplomat from a non-Western country told AFP on condition of anonymity.

Another two diplomats said this was the first time that Nackaerts, who is retiring in September, had said this.

Previously the IAEA has said that work seen from space at Parchin, including moving large quantities of earth, has "seriously undermined (its) ability to conduct effective verification."

The agency says that these "extensive activities" began in January 2012 -- after seven years of virtually no activity -- in and around a building containing a "large containment vessel" it had identified the previous March.

It still wants to visit, however, believing it may still be able to find evidence of nuclear materials, and experts believe removing radioactive traces is impossible.

"The chances of wiping out all traces are slim," one senior Western diplomat said earlier this week.

Information that the IAEA says was provided to it by member states indicated that Iran used the vessel for "hydrodynamic experiments", which would be "strong indicators of possible nuclear weapon development".

The IAEA's latest quarterly report on Iran, released last week, said that in the previous three months Iran had "conducted further spreading, levelling and compacting of material over most of the site, a significant proportion of which it has also asphalted."

A source familiar with the investigation said last week that Iran has also dumped rubble from Parchin in nearby lakes. The IAEA report also said that there have also been "indications of activity within the chamber building".

Iran denies ever having conducted nuclear weapons research, including at Parchin, and says that as a non-nuclear site the IAEA has no right to demand inspections there. It is also highly sensitive to allowing outsiders access to a military base.

It also says that the agency visited twice in 2005 and that evidence of any clean-up is "childish", but the IAEA says more information made available to it since then makes it wants to go back.

The allegations concerning Parchin form part of a major report issued by the IAEA in November 2011 summarising information of suspected nuclear weapons research that it had been given.

In 10 fruitless meetings since then the IAEA has pressed Iran for access to documents, sites and scientists involved in these alleged activities, which the agency believes were carried out mostly before 2003 and possibly since.

Iran complains that most of the IAEA's findings are based on intelligence from the likes of the CIA and Israel's Mossad, intelligence that it complains it has not been allowed to see.

It says it will only agree to grant access to Parchin as part of a "structured approach" deal covering all issues, but IAEA head Yukiya Amano called in March on Iran to let it into Parchin without waiting for this wider accord.

The IAEA fears that through the "structured approach" Iran wants to limit the agency's future inspection rights by for example agreeing to "close" an issue for good once it has been addressed.