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Each time they are caught, the Iranians belatedly inform the IAEA and claim they are not breaking the rules, he said, and the IAEA then agrees to monitor the new facility.

“What you’re doing is increasing the noise-to-signal ratio and making it much tougher to find the next covert facility,” he said.

Mr. Sokolski agreed, however, that the timing of the disclosure was “fortuitous.” And he gave credit to the Obama administration “for keeping its powder keg dry.”

White House officials acknowledged that the Iranians will determine when IAEA inspectors will be allowed into the site. IAEA spokesperson Marc Vidricaire said that the organization in Vienna “has requested Iran to provide specific information and access to the facility as soon as possible.”

Mr. Obama said that the intelligence on the once secret site, tunneled into mountains 18 miles north of Qom, which is about 160 miles southwest of Tehran, was the product of U.S., British and French intelligence, had been “thoroughly scrubbed” and was “solid.”

“The size and configuration of this facility is inconsistent with a peaceful program,” he said.

“The facility is located in an underground tunnel complex on the grounds of an Islamic Revolutionary Guard Corps base,” an official in Washington said. “The site is under the management of the Atomic Energy Organization for Iran, but is unknown to all but the most senior AEOI officials.” He spoke on the condition that he not be named because of the sensitivity of the information.

White House officials here explained that the site was configured for 3,000 centrifuges - not enough to produce fuel for a civilian reactor, which Iran claims is the goal of its program, but big enough to make highly enriched uranium for one or two nuclear bombs a year.

In 2002, an Iranian opposition group revealed Iran’s first secret enrichment plant, at Natanz. It now has about 8,000 centrifuges under IAEA supervision producing low-enriched uranium.

A 2007 U.S. intelligence estimate said Iran appeared to have stopped work on nuclear warheads but was continuing enrichment, which can produce fuel for civilian reactors or bombs.

“It was evident to everybody, both the United States and our allies, that if the Iranians wanted to pursue a nuclear weapons option, the use of the Natanz facility was a very unattractive approach. … So the obvious option for Iran would be to build another secret underground enrichment facility,” said a senior White House official.

“Our intelligence services, working in very close cooperation with our allies, for the past several years have been looking for such a facility. And not surprisingly we found one. So we have known for some time now,” he said.

White House officials told reporters that they briefed Russian, Chinese and German officials about the technical details of their evidence on the sidelines of global economic talks here. They said that Mr. Obama told Mr. Medvedev about the intelligence - shared with the British and French for some time - at their meeting in New York on Wednesday.

That sheds light on why American officials who briefed the press afterward seemed so elated with a statement from the Russian leader that sanctions were sometimes “inevitable.”

The Kremlin released a statement Friday saying that “Iran’s construction of a uranium enrichment plant violates decisions of the United Nations Security Council” and calling for the IAEA to “investigate this site immediately.”

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