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 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.
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”.