Satellite image provided by GeoEye in September 2009 showing facility under construction inside a mountain some 20 miles (32km) north-east of Qom, Iran
The International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, says Iran has doubled production capacity at the Fordo nuclear site.
Its latest quarterly report also said Iran had "significantly hampered" its ability to inspect the Parchin military site, reports the BBC.
Iran has produced 189kg (417 lb) of higher-grade enriched uranium since 2010, up from 145kg, it added.
Iran denies its nuclear programme has any military aspect.
The number of enrichment centrifuges at Fordo, at a facility buried deep inside a mountain near the holy city of Qom, had more than doubled to 2,140 from 1,064 in May, the IAEA said.
However, the new machines were not yet operating, it said.
Iran says the aim of the Fordo site is to enrich uranium for civilian use up to a maximum of 20%.
In May, UN nuclear inspectors found traces of uranium enriched at 27% at the site, but Iran said those readings could be accidental.
Analysts say 27% would bring Iran closer to making weapons-grade uranium.
In Thursday's report, the IAEA also said the Parchin site had been "sanitised" and that Iran had "been conducting activities at that location that will significantly hamper the agency's ability to conduct effective verification", if inspectors were granted access.
The Parchin site is suspected of being used for experiments related to nuclear weapons.
The overall Parchin complex is one of Iran's leading munitions centres - for the research, development and production of ammunition, rockets and high explosives.
Iran says it needs nuclear material for energy and medical needs.