The existence of the 450-hectare facility was first revealed to the Vienna-based International Atomic Energy Agency (IAEA) in 2002 by an Iranian exile group. Yesterday marked the first time reporters have been allowed to photograph Natanz. At its heavily guarded gate, there were no signs to indicate the nature of the work going on inside.
Washington and the European Union fear Iran could be using nuclear centrifuges at Natanz and elsewhere to produce heavily enriched uranium for nuclear weapons.
Khatami, who accompanied the tour, admitted that Tehran plans to enrich uranium as part of what he calls a "pilot program" at Natanz. But he repeated Tehran's long-held assertion that its nuclear program is only for generating electricity.
"We will definitely enrich [uranium]. And naturally we will start with a pilot [program]," he said. "I hope that this step will be taken with an agreement -- an understanding and commitment from our European friends and the IAEA regarding our commitments, which we have met."
The tour was an unusual gesture of openness by Iran. The journalists were taken deep inside a building where, two levels below ground, they were shown a vast, empty room designed for 50,000 enrichment centrifuges.
Iranian officials say the enrichment facility was built more than 18 meters underground because of what they call "security problems."
Ian Kemp, a London-based independent defense expert, says it is a precaution against possible aerial attack by the United States or Israel -- which both have vowed to stop Iran from acquiring nuclear weapons.
"From the Iranian perspective," Kemp says, "they would be justified in taking defensive measures, not only for their nuclear facilities but also for their non-nuclear power generation facilities. They've experienced in the past that Israel has the capability to strike targets inside Iran. And, of course, there was the 2003 campaign [by the United States in Iraq]. So they know that the power infrastructure would be a likely target if the Americans were ever to take military action against Iran."
Centrifuges are used to purify uranium fluoride gas into fuel for reactors or bombs by spinning the radioactive material at high speeds. Low-grade enriched uranium is used in nuclear power plants. High-grade "heavily enriched" uranium is needed to make the core of a nuclear bomb.
The journalists were not shown any centrifuges. And they were not allowed to visit the pilot enrichment facility at Natanz to inspect dozens of centrifuges that were sealed off by IAEA inspectors in October 2003 pending discussions with the European Union on the future of its nuclear program.
In Washington, U.S. State Department spokesman Adam Ereli dismissed the tour of Natanz as a "staged media event" that falls short of the openness needed to end Iran's nuclear dispute with the United States and the European Union.
Ereli says if Iran is really serious about transparency in its nuclear program, it should answer all of the IAEA's outstanding questions.
He says Iran should stop denying IAEA inspectors full and unrestricted access to sites like the Parchin high-explosives facility about 30 kilometers southwest of Tehran.
And he says Tehran should stop refusing IAEA requests to interview key officials associated with Iran's nuclear activities.
Kemp believes the U.S. State Department is right to dismiss of the value of the journalists' tour.
"I think the State Department is very accurate about the usefulness of journalists -- who have very little understanding of the complexities of nuclear issues or the sort of insight they would be able to bring to an inspection of Iranian nuclear facilities," he says. "This really is something that requires experts of the International Atomic Energy Agency. Or, indeed, experts that are agreed upon by the parties that are concerned. Because, of course, much of this equipment can be used for dual purposes -- nuclear power for civilian use but also spin-off for military programs."
IAEA inspectors first visited Natanz in early 2003. Tehran is currently engaged in talks with a troika of nations from the European Union, which wants Iran to permanently scrap Natanz and other nuclear fuel work in return for assistance with developing nuclear energy and other economic and security cooperation.