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 said.

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.

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.

The letter said Iran's enrichment level would be up to 5 percent, he said. The agency has requested that Iran provide specific information and access to the nuclear facility as soon as possible.

Obama made an announcement regarding the second Iranian facility at a news conference Friday morning before the opening of the G-20 economic summit in Pittsburgh, Pennsylvania.

Several diplomatic sources told CNN they were aware of the letter.

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.

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.