An exiled Iranian opposition group claimed on Thursday to have evidence of a hidden nuclear site located in tunnels beneath a mountain near the town of Damavand, 70 kilometres (44 miles) northeast of Tehran.
The Paris-based militant group the People's Mujahedin of Iran (MEK), alleges the site has existed since 2006 with the first series of subterranean tunnels and four external depots recently completed.
The group also claims the recently elected president Hassan Rohani, a former nuclear negotiator, had a "key role" in the programme.
The Vienna-based International Atomic Energy Agency (IAEA) remained non-committal about the MEK's claims.
"The Agency will assess the information that has been provided, as we do with any new information we receive," spokeswoman Gill Tudor told AFP.
Founded in the 1960s to oppose the rule of the Shah, the MEK was considered a terrorist organisation by the United States until last year, and has provided information about the Iranian nuclear programme on several occasions.
"The organisation of the People's Mujahedin of Iran (MEK) has discovered credible evidence of a secret new nuclear site, gathered over a year by 50 sources in various parts of the regime," said a statement from the National Council of Resistance of Iran (NCRI), the umbrella group of which MEK is a part.
"The codename of the project is 'Ma'adane-e Charq' (literally 'the mine of the east') or 'Project Kossar'. This site is hidden in a series of tunnels under a mountain near the town of Damavand," it said.
The report added that Mohsen Fakhrizadeh, a senior official in Iran's Revolutionary Guard, is also a managing director of a company the MEK claims is overseeing the project's "nuclear, biological and chemical programmes."
The UN's atomic watchdog IAEA has attempted to speak to Fakhrizadeh in the past without success.
The "next phase" of the project will be the construction of up to 30 tunnels and 30 depots, the report added.
The report concluded: "These revelations demonstrate once again that the Mullahs' regime has no intention of stopping or even suspending the development of a nuclear weapon," the MEK said, calling on the IAEA to visit the secret site.
The West suspects Iran of seeking the atomic bomb, although Tehran has repeatedly insisted its nuclear programme was merely for peaceful purposes. The standoff has prompted a raft of sanctions from the UN, the United States and Europe.