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.