So far Iran has denied that there are other hidden sites in addition to the one built deep underground on a military base north of the holy city of Qum. The inspectors were given access to the half-built plant late last month and reported that they had found it in “an advanced state” of construction, but that no centrifuges - the fast-spinning machines needed to make nuclear fuel - had yet been installed.

They confirmed American and European intelligence reports that the site was built to house about 3,000 centrifuges, enough to produce enough material for one or two nuclear weapons a year. But that is too small to be useful in the production of fuel for civilian nuclear power, which is what Iran insists was the intended purpose of the site.

The plant’s existence was revealed in September, as many as seven years after construction had begun.

The report by the International Atomic Energy Agency’s inspectors came just two days after President Obama, expressing increasing impatience with Iran’s responses in nuclear negotiations, indicated that he would begin to plan for far more stringent economic sanctions against Tehran. He was joined during that announcement by Russian President Dmitri A. Medvedev, but Medvedev was vague about whether Russia was now prepared to join in those sanctions.  Obama was expected to take up the issue on Tuesday with Chinese President Hu Jintao, where President Obama is on a state visit. China, like Russia, has historically resisted sanctions on Iran.