PITTSBURGH, Pennsylvania (CNN) -- The United States, France and Britain have presented "detailed evidence" to the U.N.'s nuclear watchdog that "Iran has been building a covert uranium enrichment facility," President Obama said Friday.
Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy -- all in Pittsburgh, Pennsylvania, for a G-20 economic summit -- accused Iran of intentionally hiding its nuclear facilities from the International Atomic Energy Agency.
They threatened a stiff response if Iran fails to conform to international obligations regarding nuclear development.
Iran's newly unveiled uranium enrichment facility "is inconsistent with a peaceful (nuclear) program," Obama said. "Iran is breaking rules that all nations must follow."
He called on Iran to "take concrete steps" to demonstrate it will comply with its international obligations to ensure its nuclear program is for civilian use and not a covert weapons program.
Brown said international leaders were prepared to impose "further and more stringent" sanctions against Tehran.
The "level of deception by the Iranian government" will "shock and anger the whole international community ... and harden our resolve," he said. There is "no choice but to draw a line in the sand."
Iran "is taking the international community on a dangerous path," Sarkozy said.
Iran has acknowledged the existence of a second uranium enrichment plant in a letter sent to the International Atomic Energy Agency, a spokesman for the nuclear watchdog agency said Friday.
"I can confirm that on 21 September, Iran informed the IAEA in a letter that a new pilot fuel enrichment plant is under construction in the country," agency spokesman Marc Vidricaire said.
Iran's letter stated that it would provide "further complementary information ... in an appropriate and due time," Marc Vidricaire said. In response, the IAEA has requested that Iran provide specific information and access to the nuclear facility as soon as possible.
Several diplomatic sources told CNN they were aware of the letter.
Obama recently shared sensitive intelligence with Russia and China about Iran's nuclear facility to get the countries' leaders on board with new sanctions, senior U.S. officials revealed Friday.
Russia and China, like the United States, are both veto-wielding members of the United Nations Security Council. Both countries have long opposed additional sanctions against Iran for its nuclear program, although Russian leaders have recently said they might be willing to compromise.
The second nuclear facility, on a military base near the Shia Muslim holy city of Qom, is thought to be capable of housing 3,000 centrifuges, not enough to produce nuclear fuel to power a reactor, but sufficient to manufacture bomb-making material, a U.S. diplomatic source who read the letter told CNN.
Iran's admission comes ahead of next week's rare meeting in Geneva, Switzerland, between Iran and the five permanent United Nations Security Council members, plus Germany.
Iran's revelation of a new nuclear site could actually "strengthen their hand" as it heads into next week's talks, according to Paul Ingram, an analyst who studies Iran and nuclear non-proliferation.
"It will be seen as an indication that they are willing to play by the rules," said Ingram, the executive director of the British American Security Information Council in London, England.
He said the timing of Iran's revelation -- in between the U.N. General Assembly sessions and the October 1 meeting -- is deliberate on Iran's part.
"This will make it more difficult to persuade them to abandon enrichment," Ingram said.
U.S. and French intelligence officials have known about the facility for several months, the source said. When Iran discovered that Western nations had knowledge of the facility, it sent the letter to the International Atomic Energy Agency.
Iran claims its nuclear enrichment program is intended for peaceful purposes, but the international community accuses it of continuing to try to develop nuclear weapons capability. Before the new letter, it had acknowledged only a uranium enrichment plant in Natanz, which nuclear inspectors visited recently.
The United Nations Security Council has implemented sanctions against Iran for refusing to halt enrichment.
Iranian President Mahmoud Ahmadinejad did not mention the plant during his visit to New York this week for U.N. General Assembly sessions. He reiterated earlier claims that Iran has fully cooperated with nuclear inspectors.
Obama has already said that "serious sanctions" are a possibility if Iran fails to adequately address the nuclear issue.
Middle East analyst Meir Javendafar said it was "very significant" that Iran had come clean.
"When pressured the regime does show some sign of flexibility," said Javendafar, author of the book "The Nuclear Sphinx of Tehran: Mahmoud Ahmadinejad and the State of Iran." He said ultimately, Iran is fearful of international isolation.