Pittsburgh — President Barack Obama and the leaders of France and Britain declared Friday that the revelation of a previously secret Iranian nuclear facility puts heavy new pressure on Tehran to quickly disclose all its nuclear efforts – including any moves toward weapons development – "or be held accountable."

A defiant Iranian President Mahmoud Ahmadinejad retorted that his nation was keeping nothing from international inspectors and needn't "inform Mr. Obama's administration of every facility that we have."

French President Nicolas Sarkozy said Iran has until December to comply or face new sanctions. Before that, on Oct. 1, the Iranians are to meet with the U.S. and five other major powers to discuss a range of issues including Iran's nuclear program.

"We will not let this matter rest," said British Prime Minister Gordon Brown, who accused Iran of "serial deception."

"The Iranian government must now demonstrate through deeds its peaceful intentions or be held accountable to international standards and international law," Obama said.

Hours later, the head of Iran's nuclear program suggested U.N. inspectors will be allowed to visit the newly revealed facility. 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.

Ahmadinejad, in New York for this week's General Assembly meeting, said that pressing his country on the newly disclosed plant "is definitely a mistake." In an interview with Time magazine, he said Iran was not keeping anything from the International Atomic Energy Agency. "We have no secrecy," he said.

Iran kept the facility, 100 miles southwest of Tehran, hidden from weapons inspectors until a letter it sent to the IAEA on Monday.

The U.S. has known of the facility's existence "for several years" through intelligence developed by U.S., French and British agencies, a senior White House official said. Obama decided to gather allies to talk publicly on Friday about the project to not to let Iran have the only word, officials said. They spoke on condition of anonymity to let the statements from Obama and the leaders remain the focus.

The plant would be about the right size to enrich enough uranium to produce one or two bombs a year, but inspectors must get inside to know what is actually going on, the official said.

The three leaders, in their dramatic joint statement that overshadowed the G-20 economic summit here, hoped the disclosure would increase pressure on the global community to impose new sanctions on Iran if it refuses to stop its nuclear program.

Beyond sanctions, the leaders' options are limited and perilous; military action by the United States or an ally such as Israel could set off dangerous events in the Islamic world. In addition, Iran's facilities are spread around and well-hidden, making an effective military response logistically difficult.

The leaders did not mention military force. Sarkozy, though, ominously said, "Everything, everything must be put on the table now. We cannot let the Iranian leaders gain time while the motors are running."

Germany is one of the six powers meeting with Iran next week, and German Chancellor Angela Merkel called the revelation "a grave development."

She told reporters that Germany, Great Britain, France and the United States had consulted on the issue and agreed to a joint response. Merkel spoke separately from her counterparts, because she had been in an already-scheduled meeting with Russian President Dmitry Medvedev.

She said "we will see" about the reactions of Russia and China, which also are part of the group of six but generally more reluctant to take a firm line on Iran.

Earlier this week, Medvedev opened the door to backing potential new sanctions against Iran, speaking days after Obama's decision to scale back a U.S. missile shield in Eastern Europe that Russia strongly opposed. However, it's unclear if that will translate into action.

Medvedev's spokeswoman said Friday that the developments "cannot but disturb us." Natalya Timakova said Medvedev would talk later in Pittsburgh on it, according to the Russian news agency ITAR-Tass.

The senior administration official said Obama told Medvedev about the Iranian facility during their meeting this week in New York. The Chinese are "just absorbing these revelations," the official said.

Chinese foreign ministry spokesman Ma Zhaoxu said Beijing wants the matter settled through negotiations.

"Iran is breaking rules that all nations must follow," Obama said.

Sarkozy and Brown struck a more defiant tone. "The international community has no choice today but to draw a line in the sand," Brown said.

Ahmadinejad made no mention of the facility while attending the U.N. General Assembly in New York this week. Iran denies Western suspicions that it is enriching uranium to build a nuclear bomb and says it is only doing so for energy purposes.

However, Iran is under three sets of U.N. Security Council sanctions for refusing to freeze enrichment at what had been its single publicly known enrichment plant, which is being monitored by the IAEA.

Officials said Iran's letter to the IAEA contained no details about the location of the second facility, such as if it had started operations or the type and number of centrifuges it was running.

One official, who had access to a review of Western intelligence on the issue, said it was underground about 100 miles southwest of Tehran and contains 3,000 centrifuges. It is not yet operational but the U.S. believes it will be by next year, said a U.S. counterproliferation official, who spoke on condition of anonymity because he was not authorized to speak publicly.

U.S. intelligence believes the facility is on a military base controlled by Iran's Revolutionary Guards, according to a document that the Obama administration sent to U.S. lawmakers. It was provided to The Association Press by an official on condition of anonymity because, though unclassified, it was deemed confidential. The military connection could undermine Iran's contention that the plant was designed for civilian purposes.

"The size and configuration of this facility is inconsistent with a peaceful program," Obama told reporters.

The U.S., British and French officials provided detailed information to the IAEA on Thursday, Obama said.

An August IAEA report said Iran had set up more than 8,000 centrifuges to produce enriched uranium at the first facility, also underground and located outside the southern city of Natanz. The report said that only about 4,600 centrifuges were fully active.

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Jahn contributed to this report from Vienna. Associated Press writers Charles Babington and Michael Fischer in Pittsburgh, Nasser Karimi in Tehran, Iran, John Heilprin in New York, and Pamela Hess and Desmond Butler in Washington also contributed.