Iran on Friday denied claims by an exiled opposition group that it has built a new hidden nuclear site deep beneath a mountain near the town of Damavand in Tehran province.
"This information is false and we reject it," foreign ministry spokesman Abbas Araqchi told the Mehr news agency.
On Thursday, the People's Mujahedin of Iran said it has evidence of a new hidden nuclear site that Iran was constructing secretly.
"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'adan-e Sharq' (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 Paris-based militant group MEK alleges that the site has existed since 2006, with the first series of subterranean tunnels and four external depots recently completed.
The group also claims that president-elect Hassan Rowhani, a former nuclear negotiator, played a "key role" in the programme.
The Vienna-based International Atomic Energy Agency (IAEA) said it "will assess the information that has been provided, as we do with any new information we receive".
Iran is suspected by much of the West and arch-rival Israel of wanting to develop atomic weapons under the guise of a civilian nuclear programme, a charge Tehran strongly denies.
It has been slapped with several rounds of sanctions by the United Nations as well as Washington and the European Union for continuing to enrich uranium.
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 Iran's nuclear programme several times.