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 United Nations Security Council ban on the program, which can be used to create nuclear arms.

News that Iran had linked up two sets of centrifuges - each consisting of 164 machines connected in series - was widely expected.

The 328 centrifuges would be the vanguard of 3,000 planned for installation in the coming months.

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Iran recently finished installing piping, electrical cables and other equipment needed to begin so-called "industrial-scale" enrichment in the vast subterranean complex, which is fortified and ringed by anti-aircraft guns in the central Iranian desert.

Both the Iranian leadership and the Vienna-based International Atomic Energy Agency had said recently that Tehran would start assembling the machines this month. Still, with Tehran under UN sanctions because of its refusal to give up the program, it upped the ante in Tehran's confrontation with the international community.

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.

Diplomats said the launch of the first two cascades may be the gist of Iran's planned announcement of "significant" nuclear progress on February 11, when it crowns 10 days of celebrations marking the anniversary of its 1979 Islamic Revolution.

"Two cascades have been installed in the underground plant, but they are not running yet," said a European Union diplomat in Vienna, headquarters of the watchdog International Atomic Energy Agency (IAEA), which has inspectors at Natanz.

Speaking separately - and demanding anonymity because their information was confidential - a diplomat accredited to IAEA and a U.S. official said that the two 164-centrifuge cascades had been set up in recent days.

A diplomat from another EU country said the assembled cascades would be switched on shortly to run empty "to test the vacuum for a few days and then, if that is successful, UF6 (uranium feedstock gas) will be added".

"The Iranians appear to intend to have about six cascades (about 1,000 centrifuges) installed by the spring, and the rest of the 3,000 by around June," said one diplomat.

Iran plans to rig up a total of 54,000 centrifuges at Natanz over the longer term.

There was no comment from Iran. On Friday, it denied reports abroad that it had begun installing the 3,000 centrifuges.

The likely next step was dry testing - running the linkups without any 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.