Iran may be "erasing" evidence of nuclear weapons work at its Parchin military complex near Tehran, according to a Washington-based research institute´s analysis of satellite imagery • The U.N.´s nuclear watchdog has expressed interest in inspecting the site, which Iran has not allowed.

A U.S. security institute says commercial satellite imagery shows new activity at an Iranian military site, raising concern that the Islamic republic may be "washing" a building that the UN´s nuclear agency wants to inspect.

Iran dismissed the report, as it has previously rejected allegations about the Parchin complex, where the UN´s International Atomic Energy Agency suspects research relevant to the production of nuclear weapons may have taken place.

"They are joking with our nation," Iranian Foreign Ministry spokesman Ramin Mehmanparast was quoted as saying by the Iranian Students´ News Agency. It is not possible to "wash" nuclear activities, he said.

Iran has yet to allow the IAEA to visit the facility southeast of Tehran, despite repeated requests.

IAEA chief Yukiya Amano reiterated last week that the agency had recently noticed some "activities" there. He gave no details but Western diplomats suspect Iran may be cleaning the site before any inspection. Tehran denies this.

The Institute for Science and International Security, a Washington- based think tank specializing in nuclear proliferation, said it had acquired commercial satellite imagery from April 9 that supports the IAEA´s concern.

"The new activity seen in the satellite image occurred outside a building suspected to contain an explosive chamber used to carry out nuclear weapons related experiments," the institute wrote on its website in a May 8 report that includes the satellite image.

The images showed items lined up outside a building and what appeared to be a stream of water, ISIS said. "The items visible outside the building could be associated with the removal of equipment from the building or with cleansing it," the statement said.

Iran´s mission to the IAEA has previously dismissed allegations aired about Parchin as "childish" and "ridiculous."

U.S. sees Iranian obstruction

"The stream of water that appears to emanate from the building raises concerns that Iran may have been washing inside the building, or perhaps washing the items outside the building," ISIS said.

Previous satellite images from recent months did not show any similar activity at the building, indicating it is not a regular occurrence, it added.

The IAEA has said that gaining access to Parchin is a priority when it holds a new round of talks with Iran in Vienna next week after two previous meetings in Tehran failed to make any notable progress.

But Western diplomats said they would be surprised if Tehran granted the request. Iran has suggested a broader agreement on future cooperation with the IAEA must be reached before it will consider letting inspectors into the site.

Western powers suspect Iran is seeking to develop the capability to make nuclear bombs. Iran, one of the world´s largest oil producers, says its program is peaceful.

An IAEA report late last year revealed a trove of intelligence pointing to research activities in Iran of use in developing the means and technologies needed to assemble nuclear weapons should it decide to do so.

One finding in the report was information that Iran in 2000 had built a large containment chamber at Parchin in which to conduct high- explosives tests that the IAEA said are "strong indicators of possible weapon development."

A senior U.S. official said on Tuesday that Iran must cooperate with the IAEA´s investigation and provide access to relevant sites, personnel and documents.

"Iran continues to delay and obstruct that process," Thomas Countryman, assistant secretary for international security and nonproliferation, told a meeting in Vienna.