The United States believes Iran's nuclear weapons program has taken a disquieting move forward with the building of two large nuclear facilities, U.S. officials said on Thursday.The disclosure raises a new challenge for President Bush as he tries to head off North Korea's nuclear weapons program as well as what Washington believes is an effort to build a nuclear weapon in Iraq.

The officials, who spoke on condition of anonymity, said a nuclear facility near the Iranian town of Natanz, and one near the town of Arak, were seen in commercial satellite photographs taken in September.

The facilities are of a type that suggest Iran could be using them to build a nuclear weapon, the officials said.

"It is true that there are two suspicious facilities in those locations in Iran. They were first publicly identified by an Iranian opposition group this past summer. They certainly are worth looking into."

Inspectors for the International Atomic Energy Agency have sought access to the two facilities but have yet to obtain it.

In New York, a U.N. official said Tehran had informed the Vienna-based International Atomic Energy Agency that it was "new facilities related to its civilian nuclear programs."

The IAEA quickly requested more information and a tour of the site, but "so far we have not been invited to the site," said the official, who asked not to be identified.

"The question here is whether the Iranians have already placed nuclear materials in the facilities," the U.N. official added. "If they are planning to put nuclear materials in, the IAEA expects Iran to now open talks to arrange for the materials to be placed under IAEA safeguards including an inspection regime."

The U.S. government considers Iran the most active state sponsor of terrorism. Iran, Iraq and North Korea make up what Bush has called an "axis of evil" bent on acquiring weapons of mass destruction and supporting terrorism.

Tehran has been developing a medium-range ballistic missile that experts say would be capable of hitting Israel.

The United States has also been at odds with Russia over its assistance to Iran in building a nuclear power plant at Bushehr on the Gulf coast.

SCIENTIFIC REPORT ON IRANIAN SITES

David Albright, president of the Washington-based Institute for Science and International Security (ISIS), told Reuters his organization was publishing its findings on the Iranian sites because it wanted the IAEA to inspect them.

"It appears that Iran is resisting having these sites visited, even though it says 'Come visit'. That's in fact why we released this information. We felt that there should some spotlight on this problem," he said.

Iran insists its nuclear program is devoted to generating electricity for peaceful purposes, but U.S. officials argue there can be no reason for a country with Iran's oil resources to want nuclear reactors.

ISIS reported in a briefing paper on its Web site (www.isis-online.org/) that the complex near Arak appeared to include a plant to produce heavy water, a nuclear product that can be used either in civilian reactors or in the fuel cycle for making weapons.

"There is concern that this effort to obtain a complete fuel cycle is aimed at developing the capability to make separated plutonium and highly enriched uranium, the two main nuclear explosive materials," it said.

The United States and Iran have been enemies since student militants seized the U.S. embassy in Tehran during the 1979 Iranian Islamic revolution and held 52 Americans hostage for 444 days.

Iranian President Mohammad Khatami accused the United States in October of fomenting the "worst kind of violence" on the pretext of fighting terrorism. He said goodwill gestures by Iran had not received an adequate response from Washington and called for a "change in vision" by U.S. politicians.

Bush has called for a change away from the Muslim clerics who run Iran. In July, he expressed solidarity with Iranian students who protested against the Islamic Republic.

Iran offered to help the U.S. military in emergencies during the U.S. military campaign in neighboring Afghanistan in 2001.

However, U.S. officials later showed irritation with Iran for seeking influence in western Afghanistan and allegedly giving refuge to some al Qaeda members, a charge Iran denied.

PHOTO CAPTION

A close up from a September 16, 2002 satellite image shows facilities in Natanz, Iran believed to be part of a previously unknown segment of Iran's nuclear program. The U.S. believes the Iranian nuclear weapons program has taken a disquieting move forward with the building of two large nuclear facilities in Natanz and Arak U.S. officials said on December 12. The Washington based Institute for Science and International Security(ISIS) has identified the Natanz facility as a site for possible uranium enrichment and gas centrifuges. (ISIS, DigitalGlobe via Reuters)