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.

Information obtained by the People's Mujahedeen Organization of Iran (PMOI) has revealed Iran began building the site in Abyek, about 70 miles (120 kilometers) northwest of Tehran, in 2005, the opposition members said.

"This is controlled, run and operated... by the ministry of defense," Alireza Jafarzadeh, former media spokesman for the National Council of Resistance of Iran (NCRI), told a press conference in Washington.

The People's Mujahedeen, the main group in the NCRI, is officially listed as a foreign terrorist organization in the United States, although a judge ruled in July that it should be removed from the blacklist.

Jafarzadeh said the information about the Behjatabad-Abyek site was shared this week with the US government, Congress and the UN's nuclear watchdog, the International Atomic Energy Agency.

Soona Samsami, who was US representative for the NCRI, said the Iranian authorities have so far spent 100 million dollars on the project and completed about 85 percent of the construction.

The pair presented satellite pictures of excavation work at the alleged nuclear site, which they said supports information gleaned from sources "inside the Iranian regime" and showed what they said were four entrances and a tunnel.

The mountain peak sitting atop the tunnel stands at 100 meters (330 feet), higher than the 80 meters (260 feet) nuclear experts say is required to prevent detection via radioactive emissions, Jafarzadeh said.

The mountain also reinforces the facility against any aerial bombardment, he added.

The tunnel, which is eight meters (26 feet) wide and 200 meters (650 feet) long leads deep inside the ground to three large halls designed to house centrifuge cascades used in the enrichment of uranium, he said.

The opposition members said the construction work began at a time when the Islamic Republic insisted it had halted its controversial nuclear activities.

The United States and other Western powers suspect Iran is using its nuclear enrichment program to build a bomb. Iran denies the charge, saying its atomic program is for peaceful purposes.

But the claims were met with some skepticism.

"I don't know if this site is one that they have discovered that our intelligence experts have not seen. I find that hard to believe, but we shall see," Pentagon spokesman Geoff Morrell said.

State Department spokesman Mark Toner was not aware that the Iranian opposition had contacted the administration about the claims, but told reporters US officials would study "whatever they present."

David Albright, the founder of the ISIS, said "it's just another one of their results you can't verify. It doesn't hold up very well in our minds. There's nothing in the satellite images that suggest a centrifuge plant."

He told AFP the NCRI have made "a lot of mistakes" with past claims about Iranian nuclear activity and they also "exaggerate their accomplishments dramatically."

Nonetheless, it would be worth having the IAEA visit such sites, he said.