US President Barack Obama on Saturday offered Iran "a serious, meaningful dialogue" over its disputed nuclear program, while warning Teheran of grave consequences from a united global front. "Iran's leaders must now choose - they can live up to their responsibilities and achieve integration with the community of nations. Or they will face increased pressure and isolation, and deny opportunity to their own people," Obama said in his weekly radio and Internet address. Evidence of Iran's building an underground plant to enrich uranium that could be used for an atomic bomb "continues a disturbing pattern of Iranian evasion" that jeopardizes global nonproliferation, the US president said. Ahead of Thursday's international talks with Iran in Geneva, Obama said the world "is more united than ever before" on this issue. Those negotiations, he said, "now take on added urgency." He urged Iran to open the site to international inspectors or face consequences. The chief option is tougher economic penalties, but on Friday Obama and administration officials did not rule out military action. "My offer of a serious, meaningful dialogue to resolve this issue remains open," Obama said. "But Iran must now cooperate fully with the International Atomic Energy Agency, and take action to demonstrate its peaceful intentions." Evidence of the clandestine facility was presented Friday by Obama and the leaders of Britain and France at the G-20 economic summit in Pittsburgh. The news overshadowed developments on regulating financial markets and reducing fossil fuel subsidies. Soon after, Russian President Dmitry Medvedev, at his own news conference, urged Iran to cooperate, as did Chinese Vice Foreign Minister He Yafei. He, however, did not endorse penalties against Teheran. At a news conference in New York, Iranian President Mahmoud Ahmadinejad said his country had done nothing wrong and Obama would regret his actions. "What we did was completely legal, according to the law. We have informed the agency, the agency will come and take a look and produce a report and it's nothing new," he said. Ahmadinejad said the plant - which Iranian officials say was reported to nuclear authorities as required - wouldn't be operational for 18 months. But he sidestepped a question about whether Iran had sufficient uranium to manufacture a nuclear weapon. The head of Iran's nuclear program suggested that UN inspectors may be allowed to visit the site. Ali Akbar Salehi called the incomplete facility "a semi-industrial plant for enriching nuclear fuel," but he gave no other details, according to the state news agency IRNA.