President Mohammad Khatami spent three hours inspecting the plant at the heart of Iran’s contested nuclear development program. Under international pressure, Iran suspended its work on building a centrifuge program here, but it insists it won’t give it up completely.

The European Union and United States believe Iran is building the plant to develop weapons-grade fuel, part of a program Washington insists aims at building nuclear arms. Tehran says its nuclear program is only for energy production.

The unfinished complex lies underground beneath a barren desert plain at the foot of a central Iranian mountain range. The area is ringed with anti-aircraft guns. Iran says the threat of a US or Israeli strike forced it to build underground to protect its technology.

Thirty journalists were taken underground into a concrete corridor where Khatami looked over technical documents, graphs and charts of the site.

Khatami inspected the plant’s string of centrifuges the core of the enrichment process but reporters were not allowed to see the machines or any other enrichment technology.

The complex would have been finished and inaugurated last year if Iran had not suspended work, said Mohammad Saeedi, deputy head of Iran’s Atomic Energy organization.

“Our experts are ready to resume work to compensate for lost opportunities during the freeze,” Saeedi said.

Semi-built structures that appeared uninhabited stood above ground in the 450-hectacre (1,100 acre) complex, located about 250 kilometers south of Tehran.

The visit was opened to the press to discount rumors that restricted work was under way at Natanz, Tehran radio said. Khatami visited the Natanz complex and another one in Isfahan to underscore Iran’s insistence on its right to develop the entire nuclear fuel cycle, it said.

Iran kept the plant at Natanz secret until word of its existence slipped out in mid-2002, hiking Western worries over the country’s nuclear program.

The plant’s string of centrifuges enrich uranium, a process that can produce fuel for nuclear reactors that generate electricity but also make material suitable for atomic warheads. The conversion facility in Isfahan reprocesses uranium ore concentrate, known as yellowcake, into uranium hexaflouride gas. The gas is then taken to Natanz and fed into the centrifuges for enrichment.

Iran says the program’s secrecy and its purchases of nuclear materials on the black market are not proof they aim to develop weapons, insisting they had to resort to such measures even for a peaceful program because of US sanctions and European restrictions.

But the discovery of the long-hidden activities raised European suspicions, moving them closer to the position of the United States, which wants to impose sanctions on Iran if it doesn’t eliminate its nuclear program.

Last year, Iran froze its enrichment activities in a bid to avoid UN sanctions and build confidence in negotiations with the Europeans. But it says maintaining the voluntary freeze depends on progress in talks with Britain, Germany and France.

Since 2002, Tehran has been cooperating with International Atomic Energy Agency, the UN nuclear watchdog, allowing inspections at Natanz and elsewhere.