Iran has told the UN nuclear watchdog it has a second uranium enrichment plant under construction, a belated disclosure sure to heighten Western fears of an Iranian bid for atom bombs.

The International Atomic Energy Agency said Iran had revealed the existence of the plant to IAEA Director-General Mohamed ElBaradei on Monday, just as six world powers and Iran prepare to discuss its disputed nuclear drive on October 1.

The IAEA said Iran had told ElBaradei in a letter that the plant would enrich uranium only to a level needed to generate electricity.

"The agency also understands from Iran that no nuclear material has been introduced into the facility," International Atomic Energy Agency spokesman Marc Vidricaire said.

The agency requested specific information and an immediate inspection of the plant to ensure it was for peaceful purposes.

Iran was previously known to have one enrichment plant, a vast underground hall at Natanz where it has stockpiled low-enriched uranium in a rapidly expanding operation with almost 9,000 centrifuges installed.

Iran's ISNA news agency quoted an "informed source" on Friday as confirming the reports of a second uranium enrichment plant, saying it was similar to the Natanz plant.

The Natanz plant is under daily surveillance by IAEA inspectors. Iran concealed the site and other sensitive aspects of its enrichment program from U.N. non-proliferation inspectors until the Iranian exiles blew the whistle in 2002.

It was not known how long the new plant had been under construction or planned. Iran stopped providing the IAEA advance information on nuclear site designs last year in retaliation for U.N. sanctions imposed over its nuclear campaign.

Washington said US President Barack Obama was aware of the development and would make an announcement at the G-20 summit of major industrialized nations in the US city of Pittsburgh later on Friday.

A U.S. official also confirmed a New York Times report that said Washington had been tracking the secret project for years and Obama decided to go public after Iran learned in recent weeks that Western intelligence agencies had penetrated the secrecy veiling the site.

Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy planned to call on Iran to let the IAEA inspect the site immediately, the New York Times said.

Iran is under U.N. sanctions for refusing to suspend enrichment and withholding access the IAEA needs to clarify intelligence indications that Iran has geared nuclear activity to develop atom bombs, not generating electricity as it says.

The Natanz facility has been using a 1970s vintage centrifuge, the P-1, whose design Iran obtained from the former nuclear smuggling network of Pakistani A.Q. Khan.