Leading Iranian opposition members claimed Thursday to have uncovered a secret nuclear enrichment site buried in the mountains northwest of Tehran and run by Iran's defense ministry.
But the Pentagon and independent Institute for Science and International Security expressed (ISIS) skepticism over the claims.
The dissident group said the information came from a network of sources inside Iran affiliated with the exiled opposition group the National Council of Resistance of Iran (NCRI) and the People's Mujahideen Organization of Iran (PMOI), a guerrilla movement opposed to the Iranian government.
While the NCRI in 2002 exposed Iran's uranium enrichment facility at Natanz and a heavy water facility at Arak, analysts say it has a mixed track record and a clear political agenda.
The group said the newly discovered site was intended as a facility to enrich uranium deep under a mountain near Qazvin, about 75 miles (120 km) west of Tehran, and was about 85 percent complete.
"This is certainly part of the secret weapons program. This has no peaceful intentions whatsoever," said Alireza Jafarzadeh, a former NCRI official.
This is certainly part of the secret weapons program. This has no peaceful intentions whatsoever
Alireza Jafarzadeh, a former NCRI official
The group had satellite photographs which appeared to show increasing construction activity in the area, which lies close to an existing Iranian army garrison.
But they presented no hard evidence to support the assertion that the site included a network of underground chambers designed to hold centrifuges to enrich nuclear material in violation of United Nations sanctions.
U.S. officials reacted cautiously, noting that the NCRI had made similar claims in the past. One official, speaking on condition of anonymity, said the United States already knew about the Qazvin facility.
"This facility has been under construction for years, and we've known about it for years. While there's still some ambiguity about its ultimate purpose -- not unusual for something that's still taking shape -- there's no reason at this point to think it's nuclear," the official said.
"The Iranians put military stuff in tunnels, too. People should be cautious about reaching conclusions here."
David Albright, head of the Washington-based Institute for Science and International Security, noted that the NCRI's previous assertions have sometimes turned out to be "unsubstantiated, exaggerated, or wrong."
But he nevertheless urged the International Atomic Energy Agency (IAEA) to invoke its right to demand a special inspection of the Qazvin site to test Tehran's willingness to provide more information about its nuclear program.
Diplomats with knowledge of the IAEA's Iran file said the agency received information from many sources and had to evaluate each for credibility.
"Before any assessment can be done, more facts are needed," said one source based in Vienna.
Before any assessment can be done, more facts are needed
An IAEA source
Code name "311"
The IAEA for years has been investigating Western intelligence reports indicating Iran has coordinated efforts to process uranium, stage missile tests and revamp a ballistic missile cone in a way suitable for a warhead.
Uranium enrichment can produce fuel for nuclear power plants or, if taken to a higher level, for atomic bombs.
Tehran says the intelligence is forged and its nuclear program is purely for peaceful purposes.
But Iran's record of secrecy has stoked suspicions, heightened by the February launch of higher-grade uranium enrichment of 20 percent fissile purity, bringing it closer to weapons-grade material.
Iran said in November that it had plans to build 10 more nuclear sites, but has provided little additional information.
The NCRI said the top-secret facility near Qazvin is run by Iran's Ministry of Defense and was launched in early 2005. It said the site is code named "311" and is supervised of Mohsen Fakhrizadeh-Mahabadi, who is already subject to U.N. sanctions because of his work on suspected nuclear weapons development.
Jafarzadeh said the group had determined that the site has three large underground halls, which he said were intended to house centrifuges, although there was no evidence that any had yet been installed.
He said its position, about 300 feet (100 meters) below the mountain, was intended to shield it from possible air strikes and to prevent detection of radioactive emission.
Jafarzadeh said the new information should buttress calls by the PMOI to be removed from the official U.S. list of terrorist organizations—something the State Department has been reluctant to do.