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.