PITTSBURGH -- The leaders of the U.S., France and Britain charged Iran has built a secret nuclear facility designed to give the Islamic republic the ability to build an atomic weapon, a revelation that significantly raises the stakes in the West's intensifying face-off with Tehran.

President Barack Obama made the disclosure Friday as heads of government gathered here for a G-20 summit on the world's financial and climate-change problems. U.S. officials said the discovery of the underground site, near the holy city of Qom, supported the long-held belief that Tehran is operating a second, clandestine facility to produce the highly enriched uranium used in making a nuclear bomb. Iran has a previously disclosed enrichment facility in Natanz.

President Obama said the public disclosure of the site -- which he was briefed on shortly after his election last fall -- gives Washington a new tool to unite the international community against Iran going into a pivotal face-to-face meeting with Iranian officials next Thursday in Geneva.

"Iran is breaking rules that all nations must follow, endangering the global nonproliferation regime, denying its own people access to the opportunity they deserve, and threatening the stability and security of the region and the world," Mr. Obama said. He was flanked by French President Nicolas Sarkozy and British Prime Minister Gordon Brown.

Both Russia and China, who are permanent members of the United Nations Security Council along with the U.S., Britain and France, have opposed stiffer economic sanctions on Tehran. The Western powers now hope the Qom facility will sway them.

U.S. officials said President Obama went public with the Qom information Friday because the White House learned that Tehran had got wind of the discovery and was reporting the site on its own to U.N. nuclear monitors. The officials said the U.S. shared intelligence about the existence of the Qom site with Russian President Dmitry Medvedev on Wednesday, when he met with Mr. Obama at the United Nations.

Mr. Medvedev emerged from that meeting and surprised diplomats by suggesting Russia would be more open to sanctions than in the past. On Friday he again took an unusually hard line, saying Iran must cooperate with an investigation by the U.N.'s monitors and that Russia would help the probe "by any available means," though he also told the G-20 group that Iran still needs a chance to prove its intentions for the plant are peaceful.

Western allies had already begun moving to a December deadline for sanctions. "If by December there is not an in-depth change by the Iranian leaders, sanctions will have to be taken," France's Mr. Sarkozy said Friday.

The White House now aims to win more support from China, whose officials were briefed on the new intelligence Wednesday in New York. China's foreign ministry, while calling for a probe, emphasized Friday it was still more interested in dialogue than punishment.

Iran showed no indication of compromise Friday. A defiant President Mahmoud Ahmadinejad told a news conference the new facility won't be operational for 18 months, and thus Iran hasn't violated any international reporting requirements -- a claim the U.S. and its allies refute, as does the U.N.'s nuclear watchdog, the International Atomic Energy Agency. Mr. Ahmadinejad said he would allow IAEA inspectors to visit the new site.

"What we did was completely legal, according to the law," he told reporters in New York. "At the end of the day, this is a very ordinary facility that has been set up, and it's only in the beginning stages."

Senior Iranian officials said the construction of the facility was reported on Thursday to the IAEA. The facility would be used for civilian purposes, they said, mainly the production of energy and medical tools. "With its absolute right for peaceful nuclear energy, the Islamic Republic of Iran has taken a successful new step in building a second plant for uranium enrichment," said the statement from the Iranian government, signed by the director of Iran's Atomic Energy Organization, Ali Akbar Salehi.

U.S. officials said they expected Tehran would try to persuade the IAEA that its actions were purely for civilian purposes. But the plant near Qom is "very heavily protected, very disguised," a senior administration official said.

The facility is large enough to hold around 3,000 centrifuges, not enough for commercial uranium enrichment, the official said, but "the right size" for a weapons plant that could produce enough highly enriched uranium for one to two bombs a year. Tehran is still "at least a few months, perhaps more" from installing all the centrifuges needed to process the fuel and starting operations, the official said.

U.S. intelligence officials said Friday that the earliest the facility could begin enriching uranium is 2010, and that it would take about a year to make enough of the highly enriched fuel needed for one nuclear weapon, plus time to get all the centrifuges running.

Because the new facility isn't yet up and running, analysts said Iran is no closer to achieving a nuclear weapon than previously thought. But the facility is more easily configured to produce highly enriched uranium than Iran's existing nuclear-fuel plant at Natanz, U.S. intelligence officials said.

With the IAEA monitoring that facility, the Iranians could enrich it to weapons-grade uranium only if it kicked out inspectors and monitoring cameras and significantly reconfigured the centrifuges there. "The obvious option for Iran was to build another, secret, enrichment facility," one senior administration official said. "We've been looking for such a facility, and not surprisingly, we found one."

The new facility has been under construction for several years, and the Iranians have worked hard to keep it covert, intelligence officials said, burying it deep underground at an Islamic Revolutionary Guard base managed by the Atomic Energy Association of Iran.

The last major intelligence report on Iran's nuclear program in 2007 said that Tehran would be capable of producing enough highly enriched uranium for a nuclear weapon sometime between 2010 and 2015. One intelligence official said Friday that while they are constantly reassessing that projection, "we haven't changed our assessment yet."

Write to Jonathan Weisman at jonathan.weisman@wsj.com, Siobhan Gorman at siobhan.gorman@wsj.com and Jay Solomon at jay.solomon@wsj.com