One image from May 25 showed signs that "ground-scraping activities" had taken place at the Parchin facility as well as the presence of bulldozers, according to diplomats who attended a closed-door briefing by UN nuclear agency officials.

This will probably strengthen Western suspicions that Iran is "sanitizing" the site of any incriminating evidence before possibly allowing inspectors of the International Atomic Energy Agency (IAEA) into the complex.

"They clearly think they have something to hide," one Western envoy said.

The Parchin complex is at the centre of Western suspicions that Iran has been developing a nuclear weapons capability despite Tehran's repeated denials of any such ambition.

Last week, the IAEA said in a report issued to member states that satellite images showed "extensive activities" at the facility southeast of Tehran - in what diplomats said was an allusion to suspected cleaning there.

At Wednesday's briefing for diplomats accredited to the Vienna-based agency, IAEA Deputy Director General Herman Nackaerts presented satellite images from November, April and earlier this month, participants said.

The May 25 image indicated that two or more small side buildings - adjacent to the main structure that is of interest to the UN agency - had been removed, diplomats said. One said this suggested that "serious work" was being carried out there.

Nackaerts did not elaborate on what he believed was happening at the site, apart from reiterating that the agency needed to go there to clarify the issue, diplomats said.

Iran, which denies Western accusations it is seeking to develop nuclear weapons capability, has dismissed charges aired about Parchin as "childish" and "ridiculous."

Earlier this week, Iran's state-run TV and semi-official Fars news agency both cited
the head of the country's nuclear agency Fereidoun Abbasi as saying that Iran had not been convinced of the need to inspect the Parchin site, adding that "no documents or reason has been presented to us" to persuade Tehran otherwise.

According to the Iranian nuclear chief, the UN's nuclear watchdog "is interested in visiting Parchin due to pressure from countries that want the agency to investigate the issue," reiterating Iran's refusal to stop enrichment: "We do not ask for permission from anyone to meet our country’s demands.”

“It would be better for them to negotiate with our country with regards to obtaining fuel and not ask us to stop producing fuel,” Abbasi added.

"There is no reason for us to give up enriching uranium to 20 percent because we produce this fuel only to meet our needs, no more and no less," He said.

Abbasi's remarks were made the U.S. Institute for Science and International Security (ISIS) claimed that Iran has significantly stepped up its output of low-enriched uranium and total production in the last five years would be enough for at least five nuclear weapons if refined much further.