President Barack Obama, center, British Prime Minister Gordon Brown, right, and French President Nicolas Sarkozy leave the stage after making a joint statement on Iran’s nuclear facility Friday during the G-20 Summit in Pittsburgh.

Pittsburgh ? Backed by other world powers, President Barack Obama declared Friday that Iran is speeding down a path to confrontation and demanded that Tehran quickly “come clean” on all nuclear efforts and open a newly revealed secret site for close international inspection. He said he would not rule out military action if the Iranians refuse.

Obama joined the leaders of Britain and France in accusing the Islamic republic of clandestinely building an underground plant to make nuclear fuel that could be used to build an atomic bomb. Iranian officials acknowledged the facility but insisted it had been reported to nuclear authorities as required.

“Iran’s action raised grave doubts” about its promise to use nuclear technology for peaceful purposes only, Obama told a news conference at the conclusion of a G-20 summit whose focus on world economic recovery was overshadowed by disclosure of the Iranian plant.

Obama said a telling moment could come next week when Iran meets with U.S. and other major nations to discuss the nuclear issue.

“Iran is on notice that when we meet with them on Oct. 1 they are going to have to come clean and they are going to have to make a choice” between international isolation and giving up any aspirations to becoming a nuclear power, he said. If they refuse to give ground, they will stay on “a path that is going to lead to confrontation.”

In a dramatic, early morning announcement about the secret Iranian facility, Obama said, “Iran is breaking rules that all nations must follow. The size and configuration of this facility is inconsistent with a peaceful program.”

Unbowed, Iranian President Mahmoud Ahmadinejad said his country had done nothing wrong and Obama would regret his accusations.

At a news conference in New York, Ahmadinejad said the plant wouldn’t be operational for 18 months but sidestepped a question about whether Iran had sufficient enriched uranium to manufacture a nuclear weapon. Still, he said such armaments “are against humanity, they are inhumane,” and he said anyone who pursues them “is retarded politically.”

Later Friday on CNN’s “Larry King Live,” Ahmadinejad said Iran did inform international authorities about its program and questioned what exactly Obama found fault with.

“We exceeded our commitment to the agency based on the regulations, and so is Mr. Obama really questioning why we informed the agency,” Ahmadinejad said, referring to the International Atomic Energy Agency.

The head of Iran’s nuclear program suggested U.N. inspectors would be allowed to visit the site. Ali Akbar Salehi called the facility “a semi-industrial plant for enriching nuclear fuel” that is not yet complete, but he gave no other details, according to the state news agency IRNA.