VIENNA - Iran has told the U.N. nuclear watchdog that it has a second uranium enrichment plant under construction, a belated disclosure sure to heighten Western fears of a stealthy Iranian quest for nuclear arms capability.

Iran told the International Atomic Energy Agency of the plant's existence in a letter to IAEA Director-General Mohamed ElBaradei on Monday just as six world powers and Iran prepare for rare talks on October 1 on its disputed nuclear drive.

The revelation, extending a history of Iran withholding sensitive nuclear plans from U.N. non-proliferation inspectors, may sharpen a standoff between the powers and Iran over its nuclear ambitions and give grist to Western calls to consider tougher U.N. sanctions against the Islamic Republic.

A senior White House official said President Barack Obama and the leaders of Britain and France, now at a G-20 summit of industrialized nations in Pittsburgh, would accuse Iran on Friday of concealing a sensitive plant from the IAEA for years.

Obama would demand that Iran allow an immediate IAEA inspection of the plant, the official said.

He said the nascent plant was believed to be designed for about 3,000 centrifuges for enriching uranium, nominally enough machines to produce material for one nuclear device in a year if run nonstop, although Iran does not appear to have mastered the sophisticated technology consistently yet, nuclear analysts say.

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

Confirming diplomatic leaks, the Vienna-based IAEA said Iran had told the agency of a new pilot, or experimental-level, uranium enrichment site that was not yet in operation.

IAEA spokesman Marc Vidricaire said Iran had stated that it intended to enrich uranium at the new plant, like its Natanz complex that was hidden from the IAEA until 2002, only to the 5 percent level suitable for power plant fuel.

The agency also understands from Iran that no nuclear material has been introduced into the facility, he said.

In response the IAEA has requested to Iran to provide specific information and access to the facility as soon as possible, he said, so U.N. inspectors could verify it would be used for peaceful purposes only.

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

Diplomats close to the IAEA said earlier Iran made clear no centrifuge machines had been installed at the new site, built inside a mountain around 160 km (100 miles) southwest of Tehran.

Vidricaire said Iran's letter pledged 'further complementary information will be provide in an appropriate and due time.'

FIRST ENRICHMENT PLANT OUTED BY IRAN EXILES

Iran had been known to have one enrichment plant, a vast underground hall at Natanz where it has stockpiled low-enriched uranium, potentially enough for bomb material, in a rapidly expanding operation with almost 9,000 centrifuges installed.

The Natanz plant, designed to ultimately hold 55,000 centrifuges, is under daily surveillance by IAEA inspectors.

But the Islamic Republic concealed that site and sensitive nuclear research and development activities from the IAEA for 18 years until an Iranian exile group 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.

The U.S. official said Washington has been tracking indications of the secret project for years and Obama decided to go public after Iran learned in recent weeks that Western intelligence agencies had penetrated secrecy veiling the site.

ElBaradei said earlier this month the IAEA had no concrete evidence of Iranian efforts to weaponize enrichment but could not rule this out since Iran shuns an IAEA protocol allowing snap inspections ranging beyond declared nuclear sites.

It had long been suspected that Iran was conducting enrichment work somewhere else. I think Iran disclosed it because they knew it would soon be made public by the United States, said Mark Fitzpatrick, chief non-proliferation analyst at the International Institute for Strategic Studies.

If Iran had not disclosed it I think it would have put much more pressure on them to be put under sanctions, Russia already having indicated that (more) sanctions were inevitable. This adds to the pressure on Iran, Fitzpatrick told Reuters.

The big question is, was this site planned to have been for military use and they decided to make it public and say it was for civilian use, and or was it intended for civilian use all along, and was this where they were going to put the more advanced centrifuges that they have been developing?

The Natanz facility has been using a 1970s vintage centrifuge, the P-1, whose design Iran obtained from the former nuclear smuggling ring of Pakistani A.Q. Khan. The temperamental P-1s have been operating at far less than full capacity.

But it has been experimenting for over two years with state-of-the-art models, adapted from smuggled-in components, that could enrich uranium 2-3 times faster than the P-1.

(Additional reporting by William Maclean in London and Matt Spetalnick in Pittsburgh; Editing by Charles Dick)

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