Iran is ready to double the output at its underground uranium enrichment facility, the UN nuclear watchdog says.
A leaked report by the International Atomic Energy Agency (IAEA) said there were some 2,800 centrifuges at Fordo and that Iran could soon double the number of them operating to 1,400.
The development is likely to fuel Western suspicions that Iran is seeking to develop nuclear weapons.
The government in Tehran insists the is entirely peaceful.
The IAEA also said Iran had produced about 233kg (512lb) of higher-grade enriched uranium since 2010, an increase of 43kg since August.
It used 96kg of the 20%-enriched uranium for conversion into fuel for its medical research reactor in Tehran. Experts say it would be difficult to enrich that uranium to 90%, which would be needed for a nuclear bomb.
Nuclear fuel had also been removed from the core of the nuclear reactor at Bushehr without a reason being given by Iran, the IAEA added.