Updated

New commercial satellite images suggest that Iran has demolished two buildings at a military site where it is suspected of trying to erase evidence of a nuclear arms program, a U.S. nonproliferation think tank said Thursday.

The images were published Thursday by the Washington-based Institute for Science and International Security, which 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.

The IAEA, the U.N.'s nuclear watchdog, presented some of those spy satellite images on Wednesday to the agency's 35-member nation board, prompting Iran's chief IAEA delegate Ali Asghar Soltanieh to dismiss allegations of a cleanup as "baseless."

Iran has consistently rejected accusations that it is attempting to erase traces possibly left by secret nuclear work at the Parchin military installation before granting U.N. inspectors permission to visit the facility. The Iranians contend that radioactive particles, should they exist, could not in fact be cleaned up. That is debatable, but the agency is looking for other evidence at Parchin as well, hoping to find traces left by the kind of high conventional explosives it suspects the Iranians tested there.

At stake is the threat an Iran armed with nuclear weapons could pose to its neighbors. The U.S. and Israel have indicated readiness to attack Iran if diplomacy and sanctions fail to curb its nuclear program. Both suspect that Iran is aiming to build nuclear weapons, and Israel believes it would be a prime target.

Iran insists its nuclear program is peaceful but has consistently rebuffed requests by the IAEA for access to Parchin, about 20 miles (30 kilometers) southeast of the capital, Tehran. Its refusal to grant agency experts access to sites, officials and documents sought by the IAEA has paralyzed a large-scale agency probe of suspected secret work on nuclear weapons for more than four years.

Parchin became a focus late last year after satellite images revealed the start of apparent cleanup work there shortly after IAEA first requested permission to visit the facility.

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."

The think tank, 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 the agency had in its possession said 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 AP 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.

The diplomat demanded anonymity because his information was confidential.

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, the U.S. think tank urged the Islamic Republic to give agency experts immediate access "and explain the significance of these apparent cleanup activities."

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.

Hopes that Iran would stop blocking 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 reporting a tentative deal for access to various sites, documents and scientists.

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.

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 atomic weapons research and development are just part of the international community's worries about Iran's nuclear program.

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 enrichment at a level closer to weapons-grade uranium at an underground bunker it says is safe from attack — has fed worries that it could quickly "break out" a weapons program.