VIENNA, Austria – 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 can 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 cascades of 164 centrifuges connected in series 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 until it reached the required level of enrichment: low for energy, high for the fissile core of nuclear warheads.

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 Islamic revolution.

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 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 countries such as the United States that believe it is trying to make nuclear weapons.

Iran says it wants to use the technology to generate nuclear power, but enriched uranium, the end product, can also be used for the fissile core of nuclear warheads if it is enriched to high-level weapons grade.

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.

The International Atomic Energy Agency 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 State Department did not respond directly to the report, but said Monday 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.”

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