The restricted report, obtained by The Associated Press, also noted Iran's refusal to allow inspectors to visit a heavy water reactor under construction, or related facilities, since unilaterally revising an agreement with IAEA earlier this year. Once completed, sometime in the next decade, that complex will produce plutonium, which, like enriched uranium, can be used to make nuclear weapons.

"The agency ... remains unable to make further progress in its efforts to verify certain aspects" about Iran's nuclear program, the report said. "Unless Iran addresses the long outstanding verification issues ... the agency will not be able to fully reconstruct the history of Iran's nuclear program and provide assurances about the absence of undeclared nuclear ... activities in Iran or about the exclusively peaceful nature of that program."

At the underground Natanz enrichment facility, the only site now open to full IAEA monitoring, Iran's ultimate stated goal is running 54,000 centrifuges to churn out enriched uranium - enough for dozens of nuclear weapons a year.

Uranium gas, spun in linked centrifuges, can result in either low-enriched fuel suitable to generate power, or the weapons-grade material that forms the fissile core of nuclear warheads.

Iran insists it wants the technology only to meet future power needs and argues it is entitled to enrich uranium under the Nuclear Nonproliferation Treaty.

But suspicions bred by nearly two decades of clandestine nuclear activities, including black-market acquisitions of equipment and blueprints that appear linked to weapons plans, have led to two sets of UN sanctions over its refusal to freeze enrichment.

The report also suggested that Iranian experts had ironed out many technical glitches that had caused breakdowns in experimental, smaller-scale centrifuge operations.

It said 1,312 centrifuges at Natanz were churning out small amounts of uranium enriched to 4.8 percent - suitable for power generation. Another 328 had been assembled and an additional 328 were being built as of May 13, it said.

"They now have 1,600, centrifuges - a year and a half ago they had 40 centrifuges," said the senior UN official. Iran was now able to link 164 centrifuges into an assembly capable of enrichment about every 10 days - a "notable" pace, the official said.

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