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 2,784 centrifuges at Fordo, and that Iran could soon double the number operating from 700 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 nuclear programme is entirely peaceful.
Talks this year about the uranium enrichment programme between Iran and the so-called P5+1 - the US, UK, France, China, Russia and Germany - have made little progress.
Bushehr fuel 'removed'
Iran's progress at Fordo was disclosed in a leaked report based on the findings of the IAEA's inspectors, who visit the facility regularly.
While the uranium enrichment plant is not yet fully operational - with only about 700 of the 3,000 centrifuges the facility is designed in use - experts say it could be within months.
The IAEA report said four new cascades of 174 centrifuges each "having been subjected to vacuum testing, were ready for feeding" with uranium hexafluoride (UF6) gas.
Once the new cascades were in operation, monthly production of 20%-enriched (medium-enriched) uranium would be about 25kg (55lb) per month, compared with 15kg at present, one official said.
The facility at Fordo, which is buried deep under a mountain inside a military base near the holy city of Qom, is designed to contain 16 cascades producing medium-enriched uranium, which experts say could be enriched to about 90%, or weapons-grade, in a relatively short time.
The IAEA also revealed in its report that Iran had produced about 233kg (512lb) of higher-grade enriched uranium since 2010, an increase of 43kg since August.
Earlier this year, the Atomic Energy Organisation of Iran (AEOI) used 96kg of its supply of 20%-enriched uranium for conversion into fuel for its medical research reactor in Tehran. Experts say it would be difficult to turn the fuel into weapons-grade uranium.
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.
The report also said that "extensive activities" at the Parchin military site had seriously undermined an investigation into indications that experiments related to nuclear weapons might have been carried out there. Iran is suspected of attempting to removing evidence.
Iran has denied that there are any nuclear-related activities at Parchin.
The IAEA's report concluded that the agency was "unable... to conclude that all nuclear material in Iran is in peaceful activities".