-- A U.S. nonproliferation think tank on Thursday published commercial satellite images of an Iranian military site linked to suspected secret work on nuclear arms, saying they show that two buildings there were demolished as well as other activities that strengthen U.N. suspicions of a cleanup.

The Washington-based Institute for Science and International Security provides consultancy services for U.S. government agencies focused on nonproliferation and is considered an objective source of information on Iran's nuclear program.

A senior diplomat who saw the photos displayed on the think tank's website and who is accredited to the International Atomic Energy Agency told The Associated Press they showed apparent cleanup work similar to that depicted on spy satellite photos supplied to the IAEA by member nations closely tracking Iran's nuclear activities.

He demanded anonymity because his information was confidential.

The postings of the photos come a day after the IAEA showed what the senior diplomat said were similar images made available separately to the agency's 35-member nation board. Asked about the allegations afterward, Ali Asghar Soltanieh, Iran's chief IAEA delegate, dismissed them as "baseless."

The IAEA, the U.N. nuclear watchdog, named the Parchin military installation late last year as the site of suspected tests of high explosives designed to set off a nuclear charge. Since then it has asked repeatedly for access, only to be rebuffed by Iran.

Parchin is only one link in what the IAEA says is a chain of evidence suggesting Tehran conducted extensive nuclear weapons research and development – something the Islamic Republic strenuously denies. But visiting Parchin became a top priority after satellite images revealed the start of apparent cleanup work there shortly after the IAEA's initial access request.

The IAEA expressed its latest concerns last week. Its Iran report noted that – while satellite photos had over past years shown "virtually no activity" at the site – "the buildings of interest to the agency are now subject to extensive activities that could hamper the agency's ability to undertake effective verification."

ISIS said on its website that the commercial satellite imagery from May 25 "shows that two small buildings at the same site as the suspected testing chamber have been completely razed."

"There are visible tracks made by heavy machinery used in the demolition process," it said. "Heavy machinery tracks and extensive evidence of earth displacement is also visible throughout the interior as well as the exterior of the site's perimeter."

The ISIS website also displayed a commercial satellite image taken April 9 which seems to show the two buildings in question still standing.

Diplomats at the closed IAEA meeting Wednesday who saw images that agency had in its possession said that one photo that was taken earlier this month also showed several buildings razed and extensive earth works around the site.

The senior diplomat told The Associated Press that – despite months of apparent "sanitization" – the building sheltering a metal pressure chamber where the explosions testing allegedly took place was still standing on the latest satellite images. But he said streams of what appear to be water trickling from inside indicate the chamber was being cleaned as well.

Hopes that Iran would end more than four years of stonewalling the IAEA's probe into its suspected secret weapons work at Parchin and elsewhere grew recently when agency chief Yukiya Amano returned from a trip to Tehran saying a tentative deal that will relaunch the investigation had been reached.

The senior diplomat said, however, that Iran has made no effort since that trip to resolve unspecified outstanding differences blocking final agreement on such a deal – even though Amano cited chief Iranian nuclear negotiator Saeed Jalili as saying they would be solved.

Ruediger Luedeking, the chief German delegate to the IAEA, urged Iran to honor the commitment Amano said was made.

"We hope that Iran concludes an agreement with the IAEA without delay," he told the AP. "Director General Amano was assured during his latest visit to Tehran that nothing stands in the way of this."

Alleged secret nuclear weapons research and development is only one of the international community's worries about Iran.

Separate from IAEA efforts on that issue, six world powers – the United States, Russia, China, Britain, France and Germany – are attempting to persuade Tehran to stop enriching uranium to a level that can be turned into warhead material more quickly than its low-enriched main stockpile. Their next meeting is in Moscow starting June 17.

Iran has shrugged off U.N. and other international sanctions, insisting it is enriching to low levels only to make nuclear fuel and to higher concentrations to power a research reactor and for scientific purposes. But because all enriched uranium can be further processed to weapons-grade material, Iran's nuclear secrecy – and its decision last year to start higher-level enrichment at an underground bunker it says is safe from attack – has fed worries that it could quickly "break out" a weapons program.

Noting that Iran had already razed its Lavisan-Shian site under IAEA investigation eight years ago before allowing agency experts to visit it in northeastern Tehran, ISIS urged the Islamic Republic to give agency experts immediate access "and explain the significance of these apparent cleanup activities."

Earlier on HuffPost:

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  • This May 25, 2012, photo provided by the Institute for Science and International Security, ISIS, shows the razing of two small support buildings in the Parchin military complex in Iran. It also shows heavy machinery tracks near the site. (AP Photo/ISIS, handout)

  • This May 25, 2012, left, and April 9, 2012, combination photo provided by the Institute for Science and International Security, ISIS, shows evidence of extensive earth dumping in the pond to the north of a high explosive test site and signs of fresh traffic on the road leading to the pond. (AP Photo/ISIS, handout)

  • This May 25, 2012, photo provided by the Institute for Science and International Security, ISIS, shows a wide view of the suspected high explosive testing site with extensive evidence of earth displacement activity in the Parchin military complex in Iran. (AP Photo/ISIS, handout)

  • This May 25, 2012, photo provided by the Institute for Science and International Security, ISIS, shows two small buildings intact near a suspected high explosive testing chamber.(AP/ISIS, handout)