“It’s odd, and there is no technical explanation for it,” the senior administration official said at a briefing. “There must be some other explanation.” He and other Obama administration officials declined to speculate why the Iranians would deliberately place their stockpile in a place where, with relatively little effort, Israel or another country could strike it.

The report also indicated that for the first time Iran told inspectors it was preparing to make its uranium into a metallic form  a step that can be explained by some civilian applications, but is widely viewed as necessary for making the core of an atom bomb. The report does not say what explanation the Iranians offered, if any, for the activity, other than general research and development.

Mr. Amano’s attitude toward Iran is being closely watched; some officials were concerned that he would be unwilling to confront the Iranians directly in his first months in office. But as one American official said Thursday, “It’s been clear to us that he recognizes the severity of what’s going on.”

In fact, the report detailed Iran’s past commitments to stop enriching uranium, its decision to go ahead and its refusal to comply with a series of United Nations Security Council resolutions demanding that it halt its work. It also detailed questions Iran has refused to answer about evidence that it may have worked on the difficult problems of developing a warhead small enough to fit on a missile.

“Since August 2008, Iran has declined to discuss the above issues with the agency or to provide any further information and access” to locations and scientists, the report said.

“Altogether,” it said, the accumulated evidence of weapons work and lack of explanatory cooperation “raises concerns about the possible existence in Iran of past or current undisclosed activities related to the development of a nuclear payload for a missile.”