Sept. 25 (Bloomberg) -- Iran is building a second plant for enriching uranium, described by France’s Foreign Ministry as “the latest grave breach” of United Nations resolutions aimed at curbing the country’s nuclear program.

President Barack Obama, U.K. Prime Minister Gordon Brown and French President Nicolas Sarkozy today will demand immediate access to the site by UN inspectors, saying it is an underground facility where Iran planned to secretly increase nuclear-fuel production, according to a U.S. administration official who spoke on condition of anonymity. The leaders are attending the opening of the G-20 economic summit in Pittsburgh.

Iran notified the UN’s International Atomic Energy Agency in a letter on Sept. 21 “that a new pilot fuel-enrichment plant is under construction,” Marc Vidricaire, an IAEA spokesman, said today in an e-mailed statement. The IAEA asked to enter the site “as soon as possible,” the spokesman said.

Britain and France urged the UN to consider stepping up pressure on Iran as Obama presided over a unanimous Security Council vote yesterday for a measure resolving to rid the world of nuclear arms.

Iran told the IAEA in the letter that more information about the facility “will be provided in an appropriate and due time,” the IAEA spokesman said. An unidentified Iranian official cited earlier today by the state-run Iranian Students News Agency also acknowledged the letter about the plant. No nuclear material has been introduced at the site, the IAEA cited Iran as saying. The plant’s location wasn’t given by the nuclear agency.

‘Serious Moves’

The announcement of the plant “just serves to heighten the concerns expressed in New York yesterday and reiterate the need for Iran to prove to the international community that it is ready to take action,” Emily Hands, a spokeswoman for Brown, said today in an e-mailed statement in London. “We will need to see serious moves.”

The new plant could be running as early as next year, the New York Times cited U.S. officials as saying. The plant is 100 miles (160 kilometers) southwest of Tehran, the newspaper said.

Iran is under three sets of UN Security Council sanctions over its refusal to halt enrichment, a process that can generate fuel for a nuclear reactor or the material for a bomb.

‘Critical Juncture’

The disclosure of the plant “is undoubtedly groundbreaking and comes at a critical juncture,” just before talks Oct. 1 between Iran and the UN Security Council’s five permanent members, plus Germany, said Gala Riani, Middle East analyst for London-based business intelligence and forecasting company IHS Global Insight.

The building of the new plant will “significantly strengthen Western states’ sense of entitlement to push harsher sanctions on Iran,” Riani said in an e-mail today.

The IAEA said in a report on Aug. 28 that, while Iran has increased the number of enrichment centrifuges in its nuclear facility at Natanz in central Iran, fewer of the devices are producing enriched uranium compared with a previous assessment in June. The number of centrifuges had increased to 8,308 from about 7,000. Centrifuges that are enriching uranium decreased to 4,592 from about 4,900, according to the report.

“We urge Iran to implement all measures required by the IAEA and the Security Council to build confidence,” British Foreign Secretary David Miliband said Sept. 23, reading from a statement.

The U.S., China, France, Russia and U.K., which have permanent seats and veto power on the Security Council, were joined by Germany in producing the statement at the UN General Assembly meeting. They also called on Iran to give a “serious response” in the Oct. 1 talks in Geneva.

The U.S. and allies say Iran’s nuclear program is a cover for building atomic weapons. Iran rejects the allegation, insisting that the work is peaceful and designed to generate energy for civilian purposes.

To contact the reporters on this story: Ali Sheikholeslami in London at alis2@bloomberg.net; Gregory Viscusi in Paris at gviscusi@bloomberg.net.

Last Updated: September 25, 2009 08:13 EDT