By Matt Spetalnick and Mark Heinrich
PITTSBURGH/VIENNA (Reuters) - U.S. President Barack Obama and other Western leaders accused Iran on Friday of building a secret nuclear fuel plant and demanded Tehran immediately halt what he called a "direct challenge" to the international community.
Obama went public with the charge in an appearance with British Prime Minister Gordon Brown and French President Nicolas Sarkozy at a Group of 20 summit in Pittsburgh, sharpening a standoff with Iran over its disputed nuclear program.
"It is time for Iran to act immediately to restore the confidence of the international community by fulfilling its international obligations," Obama said, adding that Tehran had been building the plant in secret for years.
The U.N. nuclear watchdog, the International Atomic Energy Agency, said earlier on Friday that Iran had just told it of a second uranium enrichment plant under construction.
The belated disclosure is sure to heighten Western fears of an Iranian bid for nuclear weapons. Tehran insists its nuclear program is for peaceful purposes to generate electricity.
A senior U.S. official, briefing reporters while Obama attended the G20 summit, said it appeared the Iranian facility was at least a few months from having all centrifuges installed and able to operate. The nascent plant was believed to be designed for about 3,000 centrifuges for enriching uranium.
At the United Nations, a senior Iranian official said accusations that the plant was clandestine were "not true."
"If it was a covert plant, we would not have informed the (International Atomic Energy) Agency," the official said.
The IAEA said Iran had disclosed the existence of the plant to IAEA Director-General Mohamed ElBaradei on Monday.
The watchdog has asked Iran to provide access to and information about the plant, built inside a mountain around 160 km (100 miles) southwest of Tehran, as soon as possible.
IAEA spokesman Marc Vidricaire said Iran had stated 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.
Mark Fitzpatrick, chief nonproliferation analyst at the International Institute for Strategic Studies, said it had long been suspected Iran was doing enrichment work at another site.
"I think Iran disclosed it because they knew it would soon be made public by the United States," he told Reuters.
"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."
The disclosure, extending a history of Tehran withholding nuclear plans from U.N. nonproliferation inspectors, gave grist to Western calls to consider tougher U.N. sanctions against Iran ahead of October 1 talks in Geneva with six world powers.
These talks will be a "serious test" of Tehran's willingness to address concerns about the new plant, the U.S. official said.
Obama shared intelligence on the Iranian plant with Russian President Dimitry Medvedev at talks in New York earlier this week, the U.S. official said.
China was only just informed about it. The U.S. official said to "stay tuned" for the Chinese position in coming days.
China had said on Thursday that more pressure would not persuade Tehran to stop the atomic weapons program it denies having. The Russians have also been reluctant to join in toughening sanctions against Tehran.
Obama accused Iran of "breaking rules that all nations must follow" and called for international inspectors "to immediately investigate this disturbing information."
Sarkozy said Iran was taking the international community down a "dangerous" path and threatened new sanctions if Iranian leaders did not change course by December.
Brown said Iran's defiance should harden the resolve of the international community, which must now "draw a line in the sand" against Tehran.
While lodging a serious accusation against Tehran, Obama said: "We remain committed to serious, meaningful engagement with Iran to address the nuclear issue through the P5+1 negotiations."
Since taking office in January, Obama has sought to engage Iran diplomatically but has been met mostly with defiance.
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 nuclear bombs, not generating electricity.
Iran's previously known enrichment plant at Natanz has been stockpiled with 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.
(Additional reporting by William Maclean in London and Matt Spetalnick in Pittsburgh; Editing by Howard Goller and John O'Callaghan)