VIENNA - Iran may have doubled its uranium enrichment capacity in an underground facility but it seems to be struggling to develop more efficient nuclear equipment that would shorten the time it would need for any atom bomb bid, experts say.

Iran's progress - or lack of it - in deploying a new generation of enrichment centrifuges is closely watched by the West as it could allow it to produce potential weapons-grade material much faster. Tehran denies this is its aim.

"Iran appears to be continuing to encounter problems in its testing of production-scale cascades of advanced centrifuges," a US think-tank, the Institute for Science and International Security (ISIS), said.

Cliff Kupchan, a Middle East analyst at consultancy Eurasia Group, said: "I note that the real game-changer, the advanced centrifuge program, still seems to be failing."

Tehran says it is refining uranium to fuel a planned network of nuclear power plants so that it can export more of its oil and gas. The United States and its allies accuse it of a covert bid to develop nuclear bombs.

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Iran has sharply increased the number of centrifuges it has in the fortified Fordow bunker, a UN report said on Thursday, showing Tehran has continued to expand its nuclear program despite Western pressure and the threat of an Israeli attack.

Iranian lawmaker Kazem Jalali said the report's publication during a meeting in Tehran of developing countries was politically motivated, the ISNA news agency reported.

The IAEA report also said buildings had been demolished and earth removed at a military site it wants to inspect. The West believes Tehran is removing evidence of illicit nuclear-linked tests, but Foreign Minister Ali Akbar Salehi said "such things can not be cleaned up or removed," ISNA quoted him as saying.

The quarterly IAEA report may strengthen a belief in Israel - which sees Iran's nuclear program as a threat to its existence - that the West's tougher economic sanctions against Tehran this year are failing to make the major oil producer curb its program.

But the International Atomic Energy Agency (IAEA) report said the newly-installed machines, which are not yet operating, were all so-called IR-1 centrifuges - a 1970s-vintage model which has been prone to breakdowns in the past.

Iran has for years been trying to introduce centrifuges with several times the capacity of the IR-1 version it now uses for the most sensitive part of its atomic activities.

If it eventually succeeded in deploying the newer models for large-scale enrichment, it could significantly reduce the time needed to stockpile refined uranium, which can be used to generate electricity or, if processed much further, nuclear explosions.

Limited Iranian knowledge

But it is unclear whether Tehran, subject to increasingly strict international sanctions, has the means and components to make the more sophisticated machines in bigger numbers.

The UN Security Council has long called on Iran to suspend uranium enrichment and Tehran's failure to comply has earned it four rounds of sanctions, as well much tougher US and European Union measures that take direct aim at its biggest export, oil.

Marking a potential step forward, Iran last year started installing more IR-4 and IR-2m models for testing at a research and development site at its enrichment facility near the central town of Natanz.

But the IAEA report suggested it was not making major progress, saying it was "intermittently" feeding uranium gas into these machines. In addition, the UN agency said Iran had yet to install three other models which it had said it would.

The IAEA report "confirms that those machines are still not ready for full-scale use," the Arms Control Association, a US-based research and advocacy group, said.

The IAEA, which regularly inspects Iran's declared nuclear sites, has little access to facilities where centrifuges are assembled and the agency's knowledge of possible centrifuge progress is mainly limited to what it can observe in Natanz.

Tehran often trumpets technical advances in its nuclear program, including the development of new centrifuges - machines that spin at supersonic speed to increase the concentration of the fissile isotope in uranium.

Early this year, President Mahmoud Ahmadinejad said Iran had a "fourth generation" centrifuge that could refine uranium three times faster than previously.

"Given the time taken for R&D (research and development) on Iran's second-generation machines, it is probably quite some time before Iran is ready to use these additional models," the Arms Control Association said in reference also to plans for even more sophisticated machines known as IR-5, IR-6 and IR6s.

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