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.

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

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 U.N. sanctions because of its refusal to give up the program, it upped the ante in Tehran's confrontation with the international community.

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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.

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. 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.

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