The United States, Britain and France on Friday demanded immediate access to a secret nuclear site revealed by Iran, dramatically heightening a showdown driven by fears Tehran is seeking atomic weapons.
In a hastily-convened appearance at the G20 summit, US President Barack Obama, French President Nicolas Sarkozy and British Prime Minister Gordon Brown threatened tough sanctions if Iran failed to open the plant to International Atomic Energy Agency (IAEA) inspections.
"We expect the IAEA to immediately investigate this disturbing information and to report to the IAEA board of governors," Obama said, styling the new plant as a "direct challenge" to international non-proliferation rules.
He said that at a meeting between Iran and world powers on October 1 in Geneva, the Islamic Republic must be ready to cooperate fully with the IAEA or face further isolation.
Obama said the secret facility had being built for several years inside a mountain near the holy city of Qom, 160 kilometers (100 miles) south of Tehran. Iran already has one enrichment plant at Natanz.
"The size and configuration of this facility is inconsistent with a peaceful program," Obama warned.
"The existence of this facility underscores Iran's continuing unwillingness to meet its obligations under UN Security Council resolutions and IAEA requirements.
"The Iranian government must now demonstrate through deeds, its peaceful intentions or be held accountable to international standards and international law."
The IAEA earlier said Iran had sent a letter on September 21 to inform the watchdog "that a new pilot fuel enrichment plant is under construction in the country," agency spokesman Marc Vidricaire said in a statement.
British Prime Minister Gordon Brown said the discovery of the plant would "shock and anger" the international community, while Sarkozy warned in his own tough remarks that Iran would face harsh sanctions by December if it did not act.
Obama, came to office offering a broad dialogue with Iran, but his offer has been shunned by the government in Tehran, but he said he remained committed to "meaningful" dialogue between Iran and world powers.
The UN Security Council has already imposed three rounds of sanction on Iran for refusing to stop uranium enrichment, a key stage in making a bomb as well as other nuclear applications.
The West accuses Iran of seeking the atomic bomb and is pressing for even tougher sanctions, but Tehran insists its activities are entirely peaceful.
Vidricaire said: "the IAEA has requested Iran to provide specific information and access to the facility as soon as possible."
The French Foreign Ministry accused Iran of committing a "serious violation" of UN resolutions with the admission of a second uranium plant. "This strengthens our suspicions," said foreign ministry spokeswoman Christine Fages.
Britain, China, France, Germany, Russia and the United States are to hold nuclear talks with Iran on the nuclear program in Geneva on October 1.
Pressure has increased with Russia indicating that it could agree to tougher sanctions.
No nuclear material has yet been introduced into the new enrichment plant under construction, the IAEA said.
Furthermore, Iran told the agency that "the enrichment level would be up to 5.0 percent," which is a low level of enrichment and not sufficiently high to make the fissile material for an atomic bomb.
Low enriched uranium is used to make nuclear fuel.
"Iran assured the agency in the letter that 'further complementary information will be provided in an appropriate and due time'," Vidricaire said.
The Natanz plant comprises a huge underground hall, under daily surveillance by IAEA inspectors, where more than 8,000 centrifuges are installed, machines that rotate at supersonic speed to refine uranium.
More than half of the centrifuges -- nearly 5,000 -- are currently actively enriching uranium.