WASHINGTON — The United States, Britain, France and Germany demanded Iran allow nuclear inspectors to visit a heretofore secret uranium enrichment plant, warning of new sanctions if Tehran did not comply.

“Iran is breaking rules that all nations must follow, ” U.S. President Obama said Friday in Pittsburgh, where he is hosting the G-20 industrial nations summit. “It is time for Iran to act immediately to restore the confidence of the international community by fulfilling its international obligations. “

British Prime Minister Gordon Brown, who joined Obama and French President Nicolas Sarkozy on the podium, threatened “further and more stringent sanctions ” should Iran not cooperate with nuclear inspectors. Sarkozy said that the tougher sanctions could come by December.

Obama said the United States joined Britain and France in presenting evidence this week to the International Atomic Energy Agency of the second nuclear plant, near the Iranian holy city of Qom. Iran maintains another nuclear facility at Natanz.

Iran, apparently anticipating this week’s revelations, on Monday wrote the IAEA acknowledging the second plant, but insisted that it was for peaceful purposes only. Obama said the second plant’s size was “inconsistent ” with such purposes and that in any case, as a signatory of the Nuclear Non-Proliferation Treaty, it was required to reveal the existence of any such facility; the Qom plant has apparently been in existence for years.

Obama said talks between Iran and the major powers would go ahead on Oct. 1, but “at that meeting Iran must be prepared to cooperate fully and comprehensivly with the IAEA. “

Iran has resisted calls to discuss its nuclear research as part of the broader negotiations aimed at reconciliation; however, this week, Mahmoud Ahmadinejad, the Iranian president, said he was ready to make Iranian scientists availavle for interviews with inspectors.

Obama said he, Sarkozy and Brown were speaking for Germany as well. Those four powers plus Russia and China make up the major powers with which Iran is negotiating. Russia and China have until now resisted sanctions, although this week Dmitry Medvedev, the Russian president, said they may be inevitable.

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