U.S. President Barack Obama said on Friday the United States preferred diplomacy in dealing with Iran over its nuclear program, but would consider imposing "sanctions that bite" if the diplomatic track did not work.

Obama, speaking at a news conference at the end of a Group of 20 summit, would not go into details about what potential sanctions might be considered. The United States and other Western countries blasted Iran on Friday over its construction of a second nuclear plant.

Obama warned Iran on Friday that it was "on notice" and said it would have to "come clean" about its disputed nuclear program at a meeting of world powers on Oct. 1.

"The international community has spoken. It is now up to Iran to respond," Obama told a news conference after a Group of 20 summit of rich and developing nations.

He said he did not want to speculate on possible courses of actions, but added: "We do not rule out any options when it comes to U.S. security interests."

Meanwhile, Russian President Dmitry Medvedev also said on Friday that if Iran does not cooperate at an Oct. 1 meeting with world powers, then "other methods" should be used to deal with Tehran's nuclear program.

Moscow has long resisted the West's campaign to level harsh sanctions against Iran, though it has recently shown greater signs of flexibility on the issue.

Earlier in the day, Obama stood with British Prime Minister Gordon Brown and French President Nicolas Sarkozy to accuse Iran of building a secret nuclear fuel plant.

Using harsh diplomatic language, the three leaders issued a strongly worded joint condemnation Friday over the existence of a second Iran uranium enrichment plant, revealed earlier this week by Tehran.

Appearing together to issue a statement ahead of the opening of the G-20 economic summit in Pittsburgh, all three condemned the actions of the Iranian government, and warned that Tehran has a limited deadline to comply with international requirements or face tighter sanctions.

Speaking first, Obama said that the three nations had presented the UN nuclear watchdog, the International Atomic Energy Agency, with detailed information regarding the second Iranian plant.

"Iran has been building a covert uranium enrichment facility... for several years," Obama said. "We expect the IAEA to immediately investigate this disturbing information."

"Iran's decision to build the nuclear plant underscores its unwillingness to meet international obligations and represents a direct challenge to the basic contract at the center of the non-proliferation agreement," he went on. "Iran is breaking rules that all nations must follow."

Sarkozy warned that Iran has until December to change its approach to international concerns about its nuclear program, or "sanctions will have to be taken."

"Everything must be put on the table now," Sarkozy said. "Iran is taking the international community on a dangerous path and we cannot let the Iranian leaders gain time while the [centrifuge] motors are running."

Brown, meanwhile, mentioned an October deadline for Iranian cooperation, and warned of the dangers of the regime's program.

"Iran's nuclear program is the most urgent proliferation challenge the world faces today," Brown said. "The level of deception by the Iranian goverment... will shock and anger the whole international community, and harden our resolve."

"we are prepared to implement further and more stringent sanctions against Iran if it doesn't comply with UN Security Council resolutions by October 1," Brown said.

Following Brown's statements, British Foreign Secretary David Miliband said on Friday "no sane person" would want a military conflict with Iran over its nuclear ambitions and he believed diplomacy could settle the dispute.

The IAEA earlier Friday demanded access to the second plant, which it was informed of by Tehran on Monday.

Two officials told the Associated Press on Friday that Iran revealed the existence of the second plant in a letter sent to International Atomic Energy Agency chief Mohamed ElBaradei.

IAEA spokesman Marc Vidricaire confirmed receipt of the letter, saying the agency was informed that a new pilot fuel enrichment plant is under construction. The letter said that the plant would not enrich uranium beyond the 5 percent level suitable for civilian energy production. That would be substantially below the threshold of 90 percent or more needed for a weapon.

Iran told the agency that no nuclear material has been introduced into the facility, he said. In response, the IAEA has requested Iran to provide specific information and access to the facility as soon as possible.

Israel and the West have warned that Iran is engaged in a secret nuclear arms program. Iran has denied this claim, maintaining that its program is purely civilian.

The officials said that Iran's letter contained no details about the location of the second facility, 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 about 160 kilometers (100 miles) southwest of Tehran and was the site of 3,000 centrifuges that could be operational by next year.

The officials who spoke to the AP - one from a European government with access to IAEA information and the other a diplomat in Vienna from a country accredited to the IAEA - demanded anonymity Friday because their information was confidential. One said he had seen the Iranian letter. The other told the AP that he had been informed about it by a UN official.

Iranian officials had previously acknowledged having only one plant - which is under IAEA monitoring - and had denied allegations of undeclared nuclear activities.

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

Ahmadinejad made no mention of the plant during his visit this week to New York for a meeting of the United Nations General Assembly, and rather stressed the fact that Iran had fully cooperated with the IAEA inspectors and that allegations of a nuclear weapons program are fabrications.

On Friday, Iran state media said the second plant was working within the framework of IAEA regulations, and is "preserving Iran's right to peaceful nuclear energy".