The UN’s nuclear watchdog says Iran has doubled production capacity at its Fordo facility.
The International Atomic Energy Agency said the number of centrifuges there have increased from just over 1,000 in May to more than 2,000 now.
Those new machines are not yet operating, the report said.
The IAEA also said Iranian officials hampered inspections of another nuclear site.
Iran denies its atomic programme has a military aspect.
Tehran says Fordo enriches uranium to 20 percent for civilian purposes.
But in May, UN inspectors found uranium enriched at 27 percent, closer to the level required to make a nuclear weapon.
Iran argues the readings are inaccurate.