A U.S. think tank published satellite imagery on Friday which it said showed "pink colored material" covering a building at a sensitive military site in Iran which UN nuclear inspectors want to visit.
The Institute for Science and International Security said the tarp - at the Parchin military complex - might be an attempt to conceal alleged clean-up work there at a time when Iran is under growing international pressure to open up its disputed nuclear program to scrutiny.
The images were consistent with a Reuters story on Thursday, which cited diplomatic sources as saying a brightly-colored tent-like structure now covered the building.
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The UN International Atomic Energy Agency (IAEA) believes that explosives tests relevant to the development of nuclear weapons were conducted in a steel chamber in the building, possibly a decade ago, and wants to go there to search for evidence.
The IAEA pressed Iran again in talks on Friday to address concerns about suspected nuclear weapons research in the Islamic state, repeating requests for access to Parchin, but their meeting ended without an agreement.
In recent months, Western diplomats have cited satellite pictures showing extensive sanitization efforts at the site in what they describe as an apparent attempt to remove any evidence of illicit nuclear activity.
Iran, which denies Western allegations that it is seeking a nuclear weapon capability, says Parchin is a conventional military facility and has dismissed allegations about it as "ridiculous."
ISIS, which tracks Iran's nuclear program closely, said commercial satellite imagery it had obtained from Aug. 15 "clearly shows the suspected high explosive test building covered in a pink-colored material.
Citing experts it had consulted, it said the size of the building had increased and that it believed that a covering tarp was supported underneath by scaffolding, adding that similar activity appeared to be taking place at a nearby building.
"The purpose of covering the buildings could be to conceal further clean-up activity from overhead satellites or to contain the activity inside," ISIS said.
"Alternatively, tarps could provide a cover for the demolition of the buildings, or portions of them, while also containing the spread of potentially contaminated debris," it added.
Iran says it must first agree a broader framework deal with the IAEA on how the agency should conduct its probe into suspected nuclear weapons research in the country before possibly allowing inspectors access to Parchin, a sprawling complex southeast of the capital Tehran.
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