CAIRO — Iran’s top nuclear official announced this weekend that the country was on the verge of starting production at its second major uranium enrichment site, in a defiant declaration that its nuclear program would continue despite new international sanctions restricting its oil revenue.

The announcement, made through official news media reports, came after a week of escalating confrontations between Washington and Tehran, including a threat that Iran would respond with military force if the United States tried to send an aircraft carrier strike group back into the Strait of Hormuz.

The imminent opening of the enrichment site — the Fordo plant, near the city of Qum — confronts the United States and its allies with difficult choices about how far to go to limit Iran’s nuclear abilities. The new facility is buried deep underground on a well-defended military site and is considered far more resistant to airstrikes than the existing enrichment site at Natanz, limiting what Israeli officials, in particular, consider an important deterrent to Iran’s nuclear aims.

When the existence of the Qum facility was first disclosed by President Obama and his counterparts in France and Britain in the fall of 2009, American officials expressed doubts that Iran would ever go forward with the facility. But once it goes into operation, the chances of disabling it, in the words of one former top Israeli official, “diminish very dramatically.”

The declaration that the facility was nearly ready came in an interview on Saturday with Fereydoon Abbasi, who was made the head of Iran’s Atomic Energy Organization shortly after surviving an assassination attempt in 2010. The official news agency Mehr quoted him as saying, “The Fordo site near Qum would soon be opened and become operational.” Iranian newspapers reported the development on Sunday.

While Iran has often exaggerated its abilities, nuclear experts say this claim is plausible. In December, inspectors for the International Atomic Energy Agency reported that during a visit to the plant they saw the finishing touches put on enrichment centrifuges and said they expected the facility to be operating soon.

Iran says its nuclear program is critical to its national security — not because it is seeking weapons, but because it wants an alternative energy source to oil and is seeking to refuel a reactor that makes medical isotopes.

More troublingly, Iran threatened early last week to close off shipping in the Strait of Hormuz, an action that analysts say could send global oil prices soaring. Iran conducted military exercises in the waterway, and then said it would use force to bar any re-entry of the United States aircraft carrier John C. Stennis and its escort ships.

While American officials and outside experts have dismissed the threat as hyperbole, and say they have every intention of patrolling the area with a carrier, there is broad concern that the Islamic Revolutionary Guard Navy could harass oil tankers passing through the narrow strait or lay mines that could create significant risks to shipping.

The opening of the plant does not significantly affect estimates of how long it could take Iran to produce a nuclear weapon, if that is its true intention. The new facility has been inspected regularly, and unless the Iranians barred inspectors or managed to deceive them, any effort to produce uranium at bomb-grade levels would most likely be detected. American officials have estimated that they would have six months to a year to react, if needed, before the enrichment was completed.

But should it come to that, the Fordo plant site itself would greatly complicate any military action. Satellite photographs show it is surrounded by antiaircraft guns, and the mountainous setting was designed to make a bombing campaign nearly impossible. Mr. Abbasi said Saturday that the plant would house a new generation of centrifuges — the machines that spin at supersonic speeds to enrich the purity of uranium — though inspectors largely saw older, far less efficient models at the plant.

“No one has a full sense of the Iranian production plan there,” said one diplomat who has studied the few details Iran has shared about the plant. “And I think that’s the point.”

Already Iran has produced enough fuel to manufacture about four weapons, but only if the fuel goes through further enrichment, nuclear experts say. Some of the fuel at Fordo, Mr. Abbasi said, would be enriched to 20 percent purity for use in a research reactor in Tehran; because of the oddities about how uranium is enriched, those batches would be the easiest to convert for use in weapons.

It is that ability that has Israel most concerned. So Israeli officials were relieved in December when Defense Secretary Leon E. Panetta, speaking at a conference in Washington, strongly suggested that the United States was determined to stop not only a weapon, but the ability to produce one.

But on Sunday, appearing on CBS’s “Face the Nation,” Mr. Panetta was less specific about how close to the line Iran would be allowed to go. Sanctions and separate embargoes against Iran were “working to put pressure on them, to make them understand that they cannot continue to do what they’re doing,” Mr. Panetta said, in comments that were taped before Mr. Abbasi’s announcement. “Are they trying to develop a nuclear weapon? No. But we know that they’re trying to develop a nuclear capability. And that’s what concerns us. And our red line to Iran is: do not develop a nuclear weapon. That’s a red line for us.”

In saying that the United States did not have any evidence that Iran was seeking to develop a nuclear weapon, Mr. Panetta was hewing closely to the conclusions the often fractious American intelligence agencies agreed upon in 2007 and again in 2010. Two National Intelligence Estimates, designed to reflect the consensus of the intelligence community, concluded that Iranian leaders had made no political decision yet to build an actual weapon. Instead, they described a series of steps that would take Iran right up to that line — and position it to assemble a weapon fairly quickly if a decision to do so were made.

When asked on “Face the Nation” about the how difficult it would be to take out Iran’s nuclear ability in a military strike, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, said: “Well, I would rather not discuss the degree of difficulty and in any way encourage them to read anything into that. But I will say that my responsibility is to encourage the right degree of planning, to understand the risks associated with any kind of military option, in some cases to position assets, to provide those options in a timely fashion. And all those activities are going on.”

Copyright © 2012 The New York Times

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