President Barack Obama and the leaders of France and Britain said Friday that the existence of a previously secret Iranian nuclear facility ups the ante on Tehran in international talks next week, declaring that Iran must cooperate on its suspected weapons development "or be held accountable."
French President Nicolas Sarkozy said Iran has until December to comply or face new sanctions.
"We will not let this matter rest," said British Prime Minister Gordon Brown, who accused Iran of "serial deception."
Said Obama: "The Iranian government must now demonstrate through deeds its peaceful intentions or be held accountable to international standards and international law."
Their dramatic three-way statement opened the G-20 economic summit here.
Obama urged Iran to fully disclose its nuclear activities and said the International Atomic Energy Agency must investigate the newly revealed site.
Iran has kept the facility, 100 miles southwest of Tehran, hidden from weapons inspectors until a letter it sent to the IAEA on Monday, which was publicly disclosed for the first time Friday.
But 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 about it after Iran's letter made clear it had become aware that Western intelligence knew of the project, 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.
Obama hopes the disclosure will 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 a dangerous chain of events in the Islamic world. In addition, Iran's facilities are spread around the country and well-hidden or buried, making an effective military response logistically difficult.
The leaders did not mention military force. But Sarkozy said ominously, "Everything, everything must be put on the table now. We cannot let the Iranian leaders gain time while the motors are running."
The disclosure comes on the heels of a U.N. General Assembly meeting at which Obama saw a glimmer of success in his push to rally the world against Iranian nuclear ambitions. And it comes just days before Iran and six world powers are scheduled to discuss a range of issues including Tehran's nuclear program.
Germany is one of those six powers, and German Chancellor Angela Merkel told reporters Friday that her country views the revelation of the second nuclear site as "a grave development" and called on Iran to answer IAEA questions about it "as quickly as possible."
She said Germany, Great Britain, France and the United States had consulted on the issue and agreed to a joint response. Merkel did not appear with Obama, Sarkozy and Brown because she had an already-scheduled meeting with Russian President Dmitry Medvedev at the same time.
She said "we will see" about the reactions of Russia and China, which also are part of the group of six but always more reluctant to take a firm line on Iran.
Earlier this week, Medvedev opened the door to backing potential new sanctions against Iran as a reward to Obama's decision to scale back a U.S. missile shield in Eastern Europe. But it's unclear if that will translate into action.
The senior administration official said Obama told Medvedev about the facility during their meeting this week in New York. The Chinese were informed about 48 hours ago and are "just absorbing these revelations," the official said.
Before the scheduling of the Oct. 1 meeting, the U.S. had long avoided direct talks with Tehran over its nuclear program.
"Iran is breaking rules that all nations must follow," Obama said.
Sarkozy and Brown struck an even more defiant tone. "The international community has no choice today but to draw a line in the sand," Brown said.
Iran's president, Mahmoud Ahmadinejad, made no mention of the facility while attending the U.N. General Assembly in New York this week. But he insisted that his country has fully cooperated with international nuclear inspectors. Iran denies that it is enriching uranium to build a nuclear bomb - as the West suspects - 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 who spoke on condition of anonymity because the information was confidential, said that Iran's letter to the IAEA contained no details about the location of the second facility, such as when - or if - it had started operations or the type and number of centrifuges it was running.
But one of the officials, who had access to a review of Western intelligence on the issue, said it was underground about 100 miles southwest of Tehran and is the site of 3,000 centrifuges that could be operational by next year. The U.S., British and French officials provided detailed information to the IAEA on Thursday, Obama said.
"The size and configuration of this facility is inconsistent with a peaceful program," he said.
Iranian semiofficial new agency ISNA on Friday confirmed reports on a second enrichment plant.
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.