President /*Barack Obama*/ and the leaders of France and Britain plan to accuse Iran of hiding the facility in an address at the opening of the G-20 economic summit Friday, a senior White House official told the AP.
The official said Obama, British Prime Minister /*Gordon Brown*/ and French President /*Nicolas Sarkozy*/ will demand Tehran open the covert facility to inspections by the International Atomic Energy Agency.
Iran is under three sets of U.N. Security Council sanctions for refusing to freeze enrichment at what had been its single known enrichment plant, which is being monitored by the IAEA.
Two officials told the AP that Iran revealed the existence of the second plant in a letter sent Monday to International Atomic Energy Agency chief /*Mohamed ElBaradei*/.
IAEA spokesman Marc Vidricaire confirmed receipt of the letter, saying the agency was informed "that a new pilot fuel enrichment plant is under construction." The letter said that the plant would not enrich uranium beyond the 5 percent level suitable for civilian energy production. That would be substantially below the threshold of 90 percent or more needed for a weapon.
Iran told the agency "that no nuclear material has been introduced into the facility," he said. "In response, the IAEA has requested Iran to provide specific information and access to the facility as soon as possible."
The officials said that Iran's letter contained no details about the location of the second facility, when - or if - it had started operations or the type and number of centrifuges it was running.
But one of the officials, who had access to a review of Western intelligence on the issue, said it was about 100 miles (160 kilometers) southwest of Tehran and was the site of 3,000 centrifuges that could be operational by next year.
The officials who spoke to the AP - one from a European government with access to IAEA information and the other a diplomat in Vienna from a country accredited to the IAEA - demanded anonymity Friday because their information was confidential. One said he had seen the Iranian letter. The other told the AP that he had been informed about it by a U.N. official.
Iranian officials had previously acknowledged having only one plant - which is under IAEA monitoring - and had denied allegations of undeclared nuclear activities.
An August IAEA report said Iran had set up more than 8,000 centrifuges to churn out enriched uranium at its cavernous underground facility outside the southern city of Natanz. The report said that only about 4,600 centrifuges were fully active.
Iran says it has the right to enrich uranium for a nationwide chain of nuclear reactors. But because enrichment can also produce weapons-grade uranium, the international community fears Tehran will make fissile material for nuclear warheads.
The IAEA says Iran has amassed more than a ton of uranium from its older Natanz centrifuges that is less than 5-percent enriched and unsuitable for weapons use. But through further enrichment, that amount would give Tehran more than enough material to produce enough weapons-grade uranium - enriched to 90 percent and beyond - for one nuclear weapon.
U.N. officials familiar with IAEA monitoring of Iran's nuclear activities have previously told the AP they suspected Iran may have undeclared enrichment plants with the state-of-the-art centrifuges that enrich more quickly and efficiently than Iran's mainstay P-1, a decades-old model based on Chinese technology.
The revelation of a secret plant further hinders the chances of progress in scheduled Oct. 1 talks between Iran and six world powers.
At that meeting - the first in more than a year - the five permanent U.N. Security Council members and Germany plan to press Iran to scale back on its enrichment activities. But Tehran has declared that it will not bargain on enrichment Iran's nuclear negotiator dismissed the threat of new sanctions in an interview released Friday.
Saeed Jalili said that Iran has "the right to uranium enrichment, and we will never give up this right," the German weekly Der Spiegel reported.
"We have lived with sanctions for 30 years, and they cannot force a great nation like the Iranian one to its knees," Jalili told Der Spiegel. "They do not scare us. On the contrary: we welcome new sanctions."
Mark Fitzpatrick, senior fellow for nonproliferation at the International Institute for Strategic Studies, suggested that Iran had little choice about disclosing the secret site ahead of the G-20 meeting in Pittsburgh.
"Iran undoubtedly announced it to the to the IAEA because they were afraid it would become known to the U.S. and others," he said.
Fitzpatrick said the disclosure "will add to the momentum behind a push for stronger sanctions on Iran" should the Oct. 1 talks in Geneva fail.
Jalili is to meet with European Union foreign policy chief Javier Solana, U.S. Undersecretary of State for Political Affairs William Burns and representatives from Britain, France, Russia, China and Germany.
On Thursday, Italian Foreign Minister Franco Frattini said the Group of Eight is giving Iran until the end of the year to commit to ending uranium enrichment - a process that can produce fissile material for the core of a nuclear weapon - and avoid new sanctions.
The existence of a secret Iranian enrichment program built on black-market technology was revealed seven years ago. Since then, the country has continued to expand the program with only a few interruptions as it works toward its aspirations of a 50,000-centrifuge enrichment facility at Natanz.
Associated Press writers Karolina Tagaris in London and Geir Moulson in Berlin contributed to this report.