The International Atomic Energy Agency says Iran has doubled the number of uranium enrichment machines it has in an underground bunker.
The agency's latest quarterly report also said Iran had "significantly hampered" the agency's ability to inspect the Parchin military site.
It says Iran has produced 189kg of higher-grade enriched uranium since 2010.
Iran denies its nuclear programme has any military aspect.
The IAEA said the number of enrichment centrifuges at a facility inside a mountain near the holy city of Qom, has more than doubled to 2,140 from 1,064 in May.
However, it said the new machines were not yet operating.
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.
The BBC reports the Parchin complex is one of Iran's leading munitions centres.