Source: Iran Assembled Nuclear Devices

Technicians have assembled two small uranium enrichment units at Iran's underground Natanz complex, diplomats and officials said Monday. The move underscored Tehran's defiance of a U.N. Security Council ban on the program, which could be used to create nuclear arms.

Speaking separately — and demanding anonymity because their information was confidential — a diplomat accredited to the Vienna-based International Atomic Energy Agency and a U.S. official said that two separate "cascades" of 164 centrifuges each had been set up in recent days.

The likely next step was "dry testing" — running the linkups without uranium gas inside — to be followed by spinning and re-spinning the gas. The process, known as enrichment, can be used to fuel nuclear power plants. But at higher levels of enrichment the material can be used in the fissile core of nuclear warheads.

The news had been widely expected. Both the Iranian leadership and the Vienna-based IAEA, which is the U.N.'s nuclear watchdog agency, had said recently that Tehran would start assembling the machines this month.

Comments last week by Iranian President Mahmoud Ahmadinejad signaled that Iran would begin the installation before Feb. 11 — the final day of nationwide celebrations in memory of the 1979 Islamic revolution. He has also called people to the streets that day to show support for the nuclear program.

In another sign that Tehran was forging ahead with plans to create a large-scale "pilot plant" — 3,000 centrifuges running in series — U.N. officials late last week told the AP that that piping, cables, control panels and air conditioning systems had been installed at Natanz to support such a number of machines.

Still, with Tehran under U.N. sanctions because of its refusal to give up the program, any decision by Iran to start assembling the so-called "cascades" ups the ante in Tehran's confrontation with the United States and other nations that believe it is trying to make nuclear weapons.

Iran says it wants to use the technology to generate nuclear power, but the U.S. and other nations believe Iran is intent on using the process to develop weapons.

A 3,000-centrifuge operation — the cornerstone of what the Iranians say will be a large-scale complex of 54,000 centrifuges — could be used to produce fissile material for two bombs a year.

IAEA officials had no comment. A U.N. official familiar with the agency's probe of Tehran's nuclear program said, however, that when IAEA inspectors last visited Natanz last week, no cascades had been assembled.

The U.S. State Department did not confirm or deny the reports Monday, saying only that it would push for "incremental" U.N. Security Council sanctions against Iran if Tehran authorities continue to ignore council demands for suspension of the country's uranium enrichment program.

Spokesman Sean McCormack said that Iran appears to be continuing "down the path of isolation."

The U.N. Security Council, which last month agreed on limited sanctions targeting people and programs linked to Iran's nuclear and missile programs, has threatened to impose further sanctions on Iran later this month if it continues to refuse to roll back its program.

Even if Tehran successfully installs 3,000 centrifuges, experts estimate it would still take several years for all of them to be running smoothly.

There has been speculation Tehran might be content to install several cascades and then temporarily freeze its activities at Natanz, hoping to be able to negotiate with a strengthened hand.

But a senior diplomat who represents his country at the IAEA said recent conversations with Iranian officials showed no signs of such a strategy.

"We declared we're going to have 3,000 and we are going to have 3,000," he said, paraphrasing an Iranian official on Tehran's plans for Natanz this year.

The International Institute for Strategic Studies, a London-based think tank, last week estimated that Iran was two to three years away from having the capacity to build a nuclear weapon. The head of U.S. national intelligence, John Negroponte, has spoken of a four-year period.

Even before starting subterranean work, Iran already had two experimental cascades of 164 centrifuges each, as well as several partially assembled cascades, all above ground at Natanz. They have been the subject or regular inspections by IAEA teams, although their authority has been restricted for a year, since Tehran withdrew broader inspecting rights after its nuclear file was referred to the U.N. Security Council.