French President Nicolas Sarkozy said Iran has until December to comply or face new sanctions. British Prime Minister Gordon Brown accused Iran of "serial deception." Obama said: "Iran is breaking rules that all nations must follow."
Their dramatic joint statement opened the G-20 economic summit.
Obama urged the International Atomic Energy Agency to investigate the site.
Iran has kept the facility, 100 miles southwest of Tehran, hidden from weapons inspectors, but the US has long known of its existence, a senior White House official told The Associated Press. Obama decided to go public with the revelation after Iran learned that Western intelligence agencies were aware of the project, the official said before the joint statement.
The officials spoke on grounds of anonymity so as not to pre-empt Obama.
But a senior Iranian official told Reuters at the United Nations that accusations that a second uranium enrichment plant under construction in Iran was clandestine were "not true". He said: "If it was a covert plant, we would not have informed the (International Atomic Energy) Agency."
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, making an effective military response difficult.
While the leaders did not mention military force, Sarkozy said, "Everything, everything must be put on the table now."
German Chancellor Angela Merkel also called on Iran to provide "all information" about the plant "as quickly as possible." She called the disclosure "a grave development that brings us to say we are concerned and expect a swift clarification."
The disclosure comes on the heels of a UN 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 days before Iran and six world powers are scheduled to discuss a range of issues including Tehran's nuclear program.
The US has long avoided direct talks with Tehran over its nuclear program.
"The Iranian government must now demonstrate through deeds its peaceful intentions or be held accountable to international standards and international law," Obama said.
Sarkozy and Brown struck a more defiant tone than their US counterpart.
"The level of deception by the Iranian government ... will shock and anger the whole international community, and it will harden our resolve," Brown declared, adding that it's time "to draw a line in the sand."
Hours before the joint appearance, a diplomat in Vienna and another European government official told The Associated Press that Tehran has informed the IAEA of a previously undeclared uranium enriching facility.
Those officials, who spoke on the condition of anonymity because the information was confidential, said Iran revealed its existence in a letter sent Monday to Mohamed ElBaradei, the head of the atomic energy agency.
Iran's president, Mahmoud Ahmadinejad, made no mention of the facility this week while attending the UN General Assembly in New York, but said that his country had fully cooperated with international nuclear inspectors.
Also at the UN meeting, Russian President Dmitry Medvedev opened the door to backing potential new sanctions against Iran as a reward to Obama's decision to scale back a US missile shield in Eastern Europe.
Iran is under three sets of UN 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.
The officials in Europe said 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 100 miles southwest of Tehran and was the site of 3,000 centrifuges that could be operational by next year.
Iranian semiofficial new agency ISNA on Friday confirmed reports on the country's second enrichment plant.
Iranian officials had previously acknowledged having only the one plant, 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 produce enriched uranium at its underground facility outside the southern city of Natanz. The report said that only about 4,600 centrifuges were fully active.
Iran says it has the right to enrich uranium for a nationwide chain of nuclear reactors. But because enrichment can also produce weapons-grade uranium, the international community fears Tehran will make fissile material for nuclear warheads.
