Iran doubles underground nuclear capacity: IAEA

The U.N. International Atomic Energy Agency also said in a report that "extensive activities" - a reference to a suspected clean-up - at Iran's Parchin military complex would hamper its investigation of possible past nuclear weapons development there, if inspectors are ever granted access.

The number of enrichment centrifuges at Fordow, a site buried deep inside a mountain to better protect it against any enemy strike, more than doubled to 2,140 from 1,064 in May, according to the IAEA's quarterly report. However, the new machines were not yet operating, it said.

The report showed that Iran had produced nearly 190 kg (418 pounds) of higher-grade enriched uranium since 2010, up from 145 kg in May.

Iran, which denies developing nuclear weapons technology, says it needs this material to fuel a medical research reactor, but it also takes it significantly closer to making potential bomb material.

The report is likely to add to Western alarm about Iran's nuclear ambitions and may further fuel speculation that Israel might launch air strikes against Iranian nuclear sites.

(Reporting by Fredrik Dahl; Editing by Robin Pomeroy)