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Mr. Amano told reporters the satellite images he has seen indicate “the use of water, demolishing buildings, removing fences and moving soil” — his first public confirmation of what others say the photos show. He suggested — without specifically saying so — that he feared the site was being cleansed ahead of any IAEA inspection.

“We have the general concern that these activities may hamper our future verification activities” at the site, he said. “Information that we have indicates that activities may have been undertaken related to the development of nuclear explosive devices and … having access is very important to clarify this issue.”

Separate from IAEA attempts to relaunch its probe, 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.

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.