VIENNA (AP) -- A U.S. institute tracking Iran's nuclear program says recent satellite images it has analyzed show further major alterations of a military site that the U.N. has long tried to access to follow up suspicions that Tehran may have used it in attempts to develop atomic arms.
The four photos from satellite company DigitalGlobe and GeoEye were seen by The Associated Press ahead of publication by the Institute for Science and International Security planned for Thursday. The images show what ISIS said was progressive asphalting of an area of the Parchin complex that the U.N.'s International Atomic Energy Agency has said was a possible location for testing conventional explosive triggers for a nuclear blast.
Experts of the U.N. nuclear watchdog organization met Iranian negotiators 10 times over 18 months in sessions ending earlier this year in futile attempts to gain access to the site and test Tehran's insistence that it was a conventional military area with no link to nuclear tests.
Iran has said the asphalting is part of regular maintenance and road work. But with its probe blocked -- and signs of other activity -- IAEA concerns have grown that it might be an attempt to cover up work on a weapons program while it keeps away inspectors.
Asphalting the area would make it more difficult to take soil samples in the search for traces of testing. Beyond the asphalt work, IAEA chief Yukiya Amano told reporters earlier this year he was also concerned about soil removal, and "possible dismantling of infrastructures" at Parchin. Because of such alleged activities, he said "it may no longer be possible to find anything even if we have access to the site."
Olli Heinonen, the previous head of the IAEA's Iran probe, also said the standoff meant any inspection by agency experts could be inconclusive even if they do get access. That, he said, meant that Tehran "has lost an important opportunity" to prove that it had nothing to hide.
But he said, without elaboration, that even without being able to inspect the facility, the IAEA has other information indicating Iranian interest in such explosive triggers and the role that the site might have played in that regard.
Heinonen suggested that the paved over area resembles a huge parking facility but said that with "very little material movements and trucks driving in and out" of the site it was "hard to see what kind of work requires such parking lots."
Iran dismisses suggestions it worked on atomic arms at Parchin or anywhere else. It has blamed the IAEA for the standoff, saying it is caused by the agency's refusal to agree on strict parameters that would govern its probe. The agency in turn says such an agreement would tie its hands by putting limits on what it could look for and whom it could question. It bases its suspicions of nuclear-weapons research and development by Iran on its own research and intelligence from the U.S., Israel and other Iran critics.
A phone call Thursday for comment to Ali Asghar Soltanieh, Iran's chief IAEA envoy, went to his voice mail.
The photos date from December 12, 2011, to Aug. 13 of this year. As seen by the AP, they show a gradually increased area of what appears to be blacktop around structures with only about a quarter remaining bare in the last image.
Alluding to earlier satellite photos indicating dismantling of buildings, apparent hosing down of the area in what the IAEA fears may be an attempt to wash away evidence, and other work, ISIS said they "clearly document activities at the Parchin site that are completely unrelated to any road-building activity."
U.S. intelligence officials say they generally stand by a 2007 intelligence assessment that asserts Iran stopped comprehensive secret work on developing nuclear arms in 2003. But Britain, France, Germany, Israel and other U.S. allies think such activities have continued past that date. That view is shared by the IAEA, which says some isolated and sporadic activities may be ongoing.
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