New revelations about Iran's nuclear ambitions were center stage at the G-20 Summit in Pittsburgh today, where President Obama, French President Nikolas Sarkozy and British Prime Minister Gordon Brown said Iran has until December to reverse course or face stiff international sanctions.
Sarkozy said that "if by December there is not an in-depth change by the Iranian leaders, sanctions will have to be taken."
Obama called evidence of Iran's second, secret underground nuclear fuel plant a "direct challenge" to the principles of nuclear nonproliferation.
"Iran has a right to peaceful nuclear power that meets the energy needs of its people," Obama said. "But Iran is breaking rules all nations must follow. ... It's time for Iran to act immediately to restore the confidence of the international community by fulfilling its international obligations.
Officials: Evidence Suggested Nuclear Bomb Site
There were three main reasons officials concluded the secret site was built to manufacture fuel for nuclear bombs, U.S. officials told ABC News.
First, though they were shown in satellite imagery, it was clear a facility was being constructed underground and heavily disguised.
Second, satellites showed the site was built on a military base and protected by armed guard around the clock.
Third, intelligence that may include spies on the ground shows the facility is designed to hold only about 3,000 centrifuges to enrich uranium, which is not enough to fuel a civilian power plant. That would need more like 50,000 centrifuges.
"It is very strong circumstantial evidence that Iran's intentions are to build a weapon," said Joe Cirincione, president of Ploughshares Fund, a foundation dedicated to security and peace funding.
The United States also has been gathering additional evidence -- including the comings and goings of equipment and suspect personnel, sources said.
Perhaps most damning is Iran's history. The nation has been working on a nuclear program since the 1980s, and has been accused of lying about it repeatedly.
Obama: Iran Must Cooperate
President Obama said Iran "must be prepared to cooperate fully" at the upcoming Oct. 1 meeting between Iranian officials and representatives of the five permanent members of the United Nations Security Council -- the U.S., Russia, China, U.K. and France -- plus Germany.
Obama said at that meeting Iran must "demonstrate that it is committed to establishing its peaceful intentions through meaningful dialogue and concrete actions."
In the meantime, Sarkozy said the "exceptional" new findings necessitate an "exhaustive, strict and rigorous investigation" by the IAEA.
"Everything, everything must be put on the table now," Sarkozy said. "We cannot let the Iranian leaders gain time while the motors are running."
Brown reiterated a sense of urgency, saying "the international community has no choice today but to draw a line in the sand. ... America, the United Kingdom, and France are one."
He said the "level of deception" by Iran in keeping its second nuclear site secret will "shock and anger the whole international community and harden our resolve."
In an exclusive meeting with Time magazine this morning as Obama, Sarkozy and Brown delivered their remarks, Iranian President Mahmoud Ahmedinejad called Obama's statement "definitely a mistake."
Ahmedinejad told Time that Iran was not keeping anything from the IAEA.
"We have no secrecy, we work within the framework of the IAEA," he said.
A U.N. spokesperson said that an Ahmedinejad press conference originally scheduled for Friday afternoon was cancelled.
International Pressure on Iran Mounts
Following a meeting with Belgium's foreign minister today, U.S. Secretary of State Hillary Clinton told reporters news of Iran's secret facility "sharpens our sense of urgency and underscores Iran's absolute need to engage seriously with us on Oct. 1 and take immediate steps to demonstrate the exclusively peaceful nature of their nuclear program."
Thursday at the U.N. Security Council, Obama, Sarkozy and Brown all pressed the need for stronger sanctions against Iran for its nuclear weapons pursuits, but they were met with some resistance from China.
On Wednesday, Obama reportedly briefed Russian President Dmitry Medvedev on the latest intelligence during a bilateral meeting, but officials say he did not discuss the secret Qom facility with Chinese President Hu Jintao.
A senior administration official says, "China is just now fully absorbing these latest revelations. I think we should stay tuned for the Chinese position in the coming days."
In a statement released Friday afternoon, the Kremlin said the existence of a secret uranium enrichment site in Iran "violates decisions of the United Nations Security Council" and called on the IAEA to "investigate this site immediately." The Russians also pledged to "assist in this investigation by any available means."
Russian President Dmitry Medvedev said earlier this week that "sanctions rarely lead to productive results, but in come cases sanctions are inevitable," a statement widely seen as indicating Russia's openness to future sanctions in this case.
The U.S. also shared its latest intelligence on Iran's nuclear program with the IAEA's chief inspector Olli Heinonen, who has reportedly approached the Iranians seeking specific information about and access to the Qom facility as soon as possible.
The "primary source intelligence" -- meaning firsthand accounts and/or satellite imaging -- provided "unambiguous intelligence," a senior administration official said, that this is a facility to enrich weapons-grade uranium.
The timing of today's announcement is directly a result of Iran realizing secrecy of the site had been breached, after which it wrote what the official called a "cursory" letter to the IAEA disclosing the site.
That prompted US, French, and British intelligence officials to go to Vienna to make their presentation to the IAEA, saying they wanted "to tell the story as it should be told."
U.S. Believes Iran Has Lied Three Times
Today's announcement is expected to galvanize pressure on Iran to allow inspectors from IAEA to see the facility and act to avert economic sanctions.
Administration officials have told ABC News that U.S. intelligence officials worked aggressively over the summer with their French and British counterparts to prepare a case to present to the IAEA this week.
"It's a highly contentious issue that, as we're seeing today, the Iranians are objecting to," a senior administration official tells ABC News. "As a general matter we wanted to make sure we have the best intelligence possible."
After the Iranians' Natanz underground facility was discovered in 2002 -- with 8,000 centrifuge machines and a stockpile of 1,400 kg of low enriched uranium -- U.S. intelligence expected Iran would try again to build another secret facility.
"Not surprisingly, we found one," an official says.
The suspected site -- described as "heavily protected and heavily disguised" -- is near Qom, Iran's holiest city, 97 miles southwest of Tehran.
Senior administration officials said the facility, not yet operational, has 3,000 centrifuge machines -- not enough to make sense for commercial use, but the "right size" if one were looking to make a little weapons-grade uranium.
According to the IAEA, Iran confessed to construction of a new pilot fuel enrichment plant on Sept. 21 in a letter to the agency, a move that some intelligence officials say indicates Iranians believed the secrecy of the site was compromised.
Iran's letter reportedly stated that "further complementary information will be provided in an appropriate and due time."
An IAEA spokesman says the agency has "requested Iran to provide specific information and access to the facility as soon as possible. This will allow the agency to assess safeguards verification requirements for the facility."
Iran denies the site is operational and says it is not producing nuclear material.
The Obama administration believes Iran has now lied to inspectors three times. In addition to today's news there were revelations in 2002 about a different clandestine plant, and news discovered in 2007 that Iran had been working to design a nuclear warhead.
ABC News' Kirit Radia and Alexander Marquardt contributed to this report.