They said the Islamic Republic told the International Atomic Energy Agency of the plant's existence in a letter to IAEA Director-General Mohamed ElBaradei earlier this week.
A senior diplomat close to the IAEA said the Iran had told the agency the facility was a pilot, or experimental-level, enrichment site that was not yet in operation.
Iran was previously known to have one enrichment plant, a vast underground hall at Natanz where it has stockpiled low-enriched uranium in a steadily expanding operation with almost 5,000 centrifuge machines.
The Natanz plant is under daily surveillance by IAEA inspectors, but Iran concealed the site and other initial aspects of its enrichment programme from U.N. non-proliferation inspectors until it was exposed by Iranian exiles in 2002.
Iran is under U.N. sanctions for refusing to suspend enrichment and failing to clarify suspicions that its nuclear activity is aimed at developing atom bombs, not generating electricity as it says.
