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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