The existence of Natanz was revealed by exiled groups several years ago
Iran has revealed the existence of a second uranium enrichment plant, the UN nuclear watchdog has confirmed.
Tehran made the announcement earlier this week in a letter to International Atomic Energy Agency (IAEA) chief Mohammed ElBaradei.
Iran has previously acknowledged it has one enrichment plant at Natanz, which IAEA inspectors are monitoring.
The US, UK and France are set to accuse Iran of concealing the plant later on Friday, media reports say.
They and other Western nations have long feared that Iran is planning to develop an atomic weapon.
IRAN'S NUCLEAR SITES
Iran insists that all its nuclear facilities are for energy, not military purposes
Bushehr: Nuclear power plant
Isfahan: Uranium conversion plant
Natanz: Uranium enrichment plant, 4,592 working centrifuges, with 3,716 more installed
Second enrichment plant: Existence revealed to IAEA in Sept 2009. Separate reports say it is near Qom, and not yet operational
Arak: Heavy water plant
Tehran has always insisted its programme is for peaceful means.
Iran is supposed to have stopped all enrichment under threat of sanctions from the UN Security Council.
News of the Iranian letter comes days before Iran is due to enter fresh talks over its controversial nuclear programme.
France called Iran's move a "serious violation" of UN Security Council resolutions.
The IAEA confirmed it received a letter from Iran on Monday informing it that "a new pilot fuel enrichment plant is under construction".
Iran told the agency that no nuclear material had been introduced into the plant, and enrichment levels would only be high enough to make nuclear fuel, not a bomb.
In response, the IAEA has requested Iran to "provide specific information and access to the facility as soon as possible", an IAEA statement adds.
Earlier, the New York Times quoted US officials as saying that the secret site - built inside a mountain near the ancient city of Qom - is not yet complete, but could be ready for operation next year.
It said the facility is believed to be capable of holding some 3,000 centrifuges, the machines that enrich uranium for electricity or, at higher levels of enrichment, for a bomb.
This development will encourage fears that Iran has other secret facilities that could be used to make a nuclear bomb, the BBC's Tehran correspondent, Jon Leyne, says in London.
BBC World Affairs correspondent Paul Reynolds
The New York Times report comes at a critical time as further sanctions on Iran, especially on its oil and gas industry, are being discussed.
If Iran is found to have hidden a plant from the IAEA, it will be put on the defensive.
Previously it had announced plans only for expanding its current enrichment plant at Natanz.
The Security Council has ordered Iran to freeze enrichment and has imposed sanctions aimed at its nuclear and ballistic missile work.
Iran has refused to comply, arguing that it has the right to develop a civil nuclear power programme.
Natanz, some 250km (150 miles) south of Tehran, had been kept secret until its existence was revealed by exiled groups several years ago, he adds.
In a report of its visit to Natanz in August, the IAEA said the site was operating 4,592 centrifuges and a further 3,716 were installed.
It said work on the first of two production halls for centrifuges, Iran had previously told inspectors it was planning, was nearly complete.
Iran has been under months of pressure to accept US President Barack Obama's offer of talks on its nuclear ambitions.
Earlier this month, Tehran agreed to "comprehensive" talks on a range of security issues - but made no mention of its own nuclear programme.
The talks are due to be held in Geneva on 1 October with Tehran and the five permanent UN Security Council members - US, UK, Russia, China and France - plus Germany.
President Obama has hinted at pursuing tougher sanctions against Tehran if progress over the crisis is not made.
Russia recently signalled it might be prepared to soften its opposition to further sanctions against Iran, although China has said such pressure would not be effective.