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 program is entirely peaceful.

Talks this year about the uranium enrichment program between Iran and the so-called P5+1 - the US, UK, France, China, Russia and Germany - have made little progress.

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, according to BBC News.