Near the town of Natanz in central Iran, 160 newly minted centrifuges stand in neat rows inside a nuclear complex that the United States and other countries were surprised to learn about only seven months ago. The machines have been tested and judged fully operational, senior Bush administration officials say. Sometime this year they will begin spinning hot uranium gas into nuclear fuel.

In a nearby building, workers are assembling parts for 1,000 more centrifuges, part of a constellation of 5,000 machines that will be linked together in a vast uranium enrichment plant now under construction. When the project is completed in 2005, Iran will be capable of producing enough enriched uranium for several nuclear bombs each year.

Details about the Natanz complex are beginning to trickle out following the first visit to the site by officials from the United Nations late last month. U.S. officials who were briefed on the visit described Iran's progress last week as "startling" and "eye-opening," so much so that intelligence agencies are being forced to dramatically shorten estimates for when Iran may acquire nuclear weapons.

But equally striking is the extent to which Iran's breakthrough caught the United States and others by surprise. For a decade, U.S. efforts to keep nuclear weapons away from Iran focused on a location far to the south, a nuclear power plant being built with Russian help near the port city of Bushehr. Fearing that Iran would extract plutonium from the reactor's fuel for a nuclear weapon, U.S. officials have imposed sanctions against Russian companies and exerted enormous pressure on Russia, China and Ukraine to prevent them from supplying the project with sensitive equipment and know-how.

All along, the Tehran government was quietly pursuing a different course, U.S. officials now say. While not foreclosing the possibility of plutonium-based bombs, they report, Iran built a clandestine and highly sophisticated nuclear infrastructure that would allow it to seek uranium-based weapons.

The Natanz plant poses a critical challenge to the Bush administration at a delicate time -- just as North Korea also appears to be intensifying its efforts to build a nuclear weapon, and on the eve of a possible war with Iraq. The disclosures will raise difficult new questions about U.S. policy and the president's declared intention to preempt threats from those who seek to acquire weapons of mass destruction.

A Secret No Longer

The pilot uranium enrichment plant at Natanz, 200 miles south of Tehran, came to light only last August, when it was exposed by an opposition group, the National Council of Resistance of Iran. While the group has been labeled by the State Department as part of a foreign terrorist organization -- the Mujahedin-e Khalq, based in Iraq -- it has often disclosed reliable information about Iran's efforts to obtain weapons of mass destruction. U.S. officials consider much of its information on the Iranian projects to be credible.

Iran already has much of the other knowledge and technology needed to become a nuclear weapons state, U.S. officials and outside experts say. Iran possesses an arsenal of medium-range ballistic missiles as well as the factories and engineers to produce new ones. It also has chemical and possibly biological weapons, according to the CIA.

"Here we suddenly discover that Iran is much further along, with a far more robust nuclear weapons development program than anyone said it had," Secretary of State Colin L. Powell said yesterday on CNN's "Late Edition." "It shows you how a determined nation that has the intent to develop a nuclear weapon can keep that development process secret from inspectors and outsiders, if they really are determined to do it."

"This," said another senior administration official, "is a country going full-bore on all aspects of the nuclear fuel cycle."

While U.S. intelligence agencies have long believed Iran was seeking a nuclear weapon, the Natanz disclosure suggests the Iranians have gone further than was previously suspected.

Iran denies having such ambitions. Iranian President Mohammad Khatami, speaking on the eve of last month's visit by the United Nations' International Atomic Energy Agency, acknowledged an ambitious nuclear power agenda, but said Iran's only interest was diversifying its energy supply for a growing population of 65 million.

As a signatory to the Non-Proliferation Treaty in good standing with the IAEA, Iran has a right to pursue nuclear technology for peaceful purposes, subject to the agency's oversight. That right includes producing its own enriched uranium. Technically, Iran was not obligated to disclose its Natanz plant to the IAEA until it began processing uranium, although Khatami pledged to give advance notice of new construction in the future.

Iran's assurances about its intentions have drawn skepticism elsewhere. Why, weapons experts ask, would a country that sits atop one of the world's largest reserves of oil and natural gas spend hundreds of millions of dollars to build nuclear power plants?

"Whether there is an economic rationale doesn't matter: This still will bring Iran within weeks of getting a large arsenal of bombs," said Henry Sokolski, a Defense Department official during the first Bush administration who served as an adviser on nonproliferation policy. "And they can do it without breaking any rules."

Iran's acknowledgment of its uranium enrichment program last month signaled a shift in the country's nuclear strategy, one that complicates the U.S. response. As long as Iran adheres to IAEA safeguards, a weapons program would be proscribed. But possessing a large amount of enriched uranium would give Iran the option of quickly launching a full-scale weapons program at a time of its choosing, either clandestinely or by renouncing its treaty obligations.

"By deciding to become transparent, Iran reduces its isolation and makes it harder for the United States or anyone else to say no to its enrichment plant," said David Albright, president of the Institute for Science and International Security, a Washington-based research organization. "At the same time it shows the world it is very close to nuclear weapons."

Advanced Infrastructure

Since the mid-1980s, Iran has made no secret of its quest for uranium enrichment. Many specialists have said Iran's final hurdle in building a bomb would be obtaining fissile material such as enriched uranium.

In December, the first satellite images of the activity in Natanz, released by the Institute for Science and International Security, appeared to verify claims by the Iranian opposition group of a large uranium enrichment plant under construction in Iran's desert interior. Why Iran would need such a plant was uncertain, as Russia had agreed to supply all the nuclear fuel needs for Bushehr, Iran's only known nuclear power reactor, which is still under construction.

The satellite photos gave little hint of what IAEA Director General Mohamed ElBaradei found at the site on Feb. 21 in the first visit by the U.N.-chartered nuclear monitoring agency.

ElBaradei discovered in one building a fully operational pilot plant equipped with the 160 gas centrifuges for enriching uranium, according to knowledgeable U.S. officials and weapons experts. An assembly room nearby contained the parts for 1,000 additional machines, about a fifth of the total number of centrifuges expected to be installed at the site. Depending on the capacities of the machines, 5,000 centrifuges could theoretically produce enough enriched uranium for at least two nuclear bombs per year.

Iran also acknowledged to the IAEA the construction of a plant to convert uranium into UF6 (uranium hexafluoride), the gaseous form of the metal that is used in centrifuges, the officials said. Some UF6 gas had already been imported by Iran from a foreign source, and some weapons experts suspect Iran enriched small amounts of uranium in a research facility elsewhere in the country. Any previous enrichment of uranium would constitute a violation of Iran's agreements under the Non-Proliferation Treaty.

It is not known precisely where Iran obtained the blueprints and the many specialized materials used to make centrifuges. Although some U.S. officials suspect Pakistan provided designs for the centrifuges in the early 1990s, the machines on display at Natanz had been significantly modified by Iranian engineers and could not be easily traced to a single country or supplier, according to U.S. and independent nuclear experts. Iran apparently acquired centrifuge motors and other parts from abroad, and it recruited foreign scientists to help master complex engineering feats, the experts said.

"The bottleneck is getting the technology -- knowing how to build machines," said Gary Samore, senior fellow at the International Institute for Strategic Studies in London. "Once a country has the technology, it's pretty hard to keep them from getting the bits and pieces.

Pointing Fingers

The National Council of Resistance of Iran, the opposition group that first reported the nuclear complex at Natanz, said that Iran continues to receive assistance from China and North Korea, among other countries. Council spokesman Alireza Jafarzadeh said Iranian officials had visited both countries in the past two years to seek technical help with uranium enrichment. More recently, two Chinese officials visited Iran to oversee mining of uranium ore near the town of Yaz, the group asserted. The report could not be independently verified.

Jafarzadeh said Western policies were partly to blame for the failure to detect Iran's secret programs and deter or eliminate them. He said U.S. and European administrations should have adopted a more forceful posture rather than seeking rapprochement with more moderate Iranian leaders, such as Khatami, whose leadership coincided with a dramatic acceleration of Iran's nuclear program.

"We warned repeatedly that if the international community would not take immediate and decisive measures, the regime could reach the point of no return with their nuclear weapons program," Jafarzadeh said. "They might well be very close to that."

But Rose Gottemoeller, a senior Energy Department official under former president Bill Clinton, said the emerging crisis over Iran was hardly due to U.S. inattention. The past three U.S. administrations were "hugely concerned" that Iran was secretly developing new capabilities, and had achieved major successes in blocking known attempts by Iran to acquire nuclear technology from Russia and China.

The Clinton administration had lobbied Russia for years to stop assisting Iran, working through a joint commission chaired by Vice President Al Gore and Russian Prime Minister Viktor Chernomyrdin. Russia at one point suspended plans to sell laser separation technology to Iran after protests from the United States. U.S. officials feared that it would be used to help Iraq enrich uranium.

Gottemoeller, now a senior associate with the Carnegie Endowment for International Peace, acknowledged that previous U.S. efforts "focused too much on the Russian connection and not enough on our relations with Iran."

Gottemoeller said the arguments for direct diplomacy were even more compelling now, if Iran is to be dissuaded from turning its new nuclear assets into weapons. North Korea recently expelled the IAEA and is pushing toward the construction of a nuclear weapon. President Bush has referred to Iran, Iraq and North Korea as an "axis of evil."

"Our three 'axis of evil' designees seem to have decided to push hard to provide themselves with weapons if they're going to be in the constant attention of the United States," Gottemoeller said. "We need a more proactive, positive way of engaging them first and then trying to shut these things down."