The People's Mujahedeen of Iran, an opposition group with an extensive network inside the country, said Tehran launched construction at the facility in 2005 and had spent $100 million (£65 million) on the tunnels.
The group has previously revealed secret atomic plants at Natanz and Qom that the Iranian regime has subsequently acknowledged to be nuclear facilities.
The exiled opposition group is a radical but deeply rooted enemy of the Islamic Republic and its armed wing Mujahedeen e-Khalk is proscribed as a terrorist organisation in the US.
"This is certainly part of the secret weapons program," said Alireza Jafarzadeh, a spokesman for the group who presented photographs of the site in Washington. "It's just moved underground, in tunnels, hidden from the outside world."
The organisation said it had already passed the information, which includes eyewitness reports from inside the facility and satellite images showing considerable development in the remote area, to the US government.
If construction was started in 2005 it would represent an embarrassing failure of US intelligence which concluded the Iranian leadership had suspended enrichment of uranium, the key component of a nuclear weapon, at the time.
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