"(The) baseless remarks about the existence of secret nuclear installations in Iran... are strongly rejected," Iran's Atomic Energy Organization Spokesman Behrouz Kamalvand told IRNA on Tuesday.
On Monday, the terrorist Mujahedin Khalq Organization (MKO) alleged its sources had confirmed the existence of the "012" military site in central Isfahan province, located inside a heavily guarded 600-meter (650-yard) tunnel.
But the group said it was unable to ascertain what was happening inside the highly secure area.
The MKO has raised several rumors on Iran's nuclear program in the past, but experts have met many of its other claims with skepticism.
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 MKO terrorist group made the allegation to influence the "positive atmosphere" in the talks.
The P5+1 -- Britain, France, the United States, Russia and China plus Germany -- and Iran continue talks in Geneva to respond to suspicions about the Iranian nuclear program.
NTJ/BA