Iran on Tuesday rejected as "baseless" claims by an exiled opposition group that it is conducting military nuclear activities in a hidden underground site, the official IRNA news agency reported.
"(The) baseless remarks about the existence of secret nuclear installations in Iran... are strongly rejected," Iran's Atomic Energy Organisation spokesman Behrouz Kamalvand told IRNA.
On Monday, the Paris-based National Council of Resistance of Iran alleged its sources had confirmed the existence of the "012" military site in central Isfahan province, located inside a heavily guarded 600-metre (650-yard) tunnel.
But the group said it was unable to ascertain what was happening inside the highly secure area.
The NCRI has revealed important aspects of Iran's nuclear programme in the past, including the existence of the Natanz facility in 2002, but experts have met many of its other claims with scepticism.
The new claim came ahead of nuclear talks between Iran and world powers in Geneva this week, after an earlier round of talks this month came close to an interim deal but stumbled on last-minute changes to a draft text.
Kamalvand said the NCRI made the allegation to influence the "positive atmosphere" in the talks.
The so-called P5+1 -- Britain, France, the United States, Russia and China plus Germany -- suspect Iran is using its nuclear programme to develop atomic weapons but Tehran insists it is purely for peaceful purposes.
The NCRI is the political umbrella for a number of dissident groups, in particular the People's Mujahedeen of Iran (MEK), founded in the 1960s to oppose the rule of the Shah.
After the 1979 Islamic revolution, the MEK took up arms against Iran's clerical rulers. It says it has now laid down its arms and is working to overthrow the Islamic regime in Tehran by peaceful means.