Washington (CNN) -- Supporters of an Iranian opposition group announced Thursday that they have "exclusive" details on a major top-secret strategic nuclear enrichment site buried deep in a mountain northwest of Tehran, but U.S. government officials and nuclear experts are not convinced.

Alireza Jafarzadeh, speaking for the National Council of Resistance for Iran, said its members obtained the information from Iran's chief opposition group, the People's Mujahedeen Organization of Iran, which was credited with exposing Iran's first nuclear site in Natanz in 2002. The PMOI is currently listed by the State Department as a designated terrorist organization.

"This has no peaceful intentions whatsoever," Jafarzadeh said at a news conference Thursday.

"This is controlled and run, (and) operated by the head of military, minister of defense, hidden from the International Atomic Energy Agency and the outside world," said Jafarzadeh, who is a former spokesman for the National Council of Resistance for Iran, which is affiliated with the PMOI.

Jafarzadeh said that Iran's Ministry of Defense has taken "extraordinary concealment measures to avoid its detection. He said the site, codenamed 311 by the Iranians, is "far more important" than a site disclosed earlier near the holy Iranian city of Qom.

Construction began in early 2005 on the site about 75 miles northwest of Tehran, near the city of Qazvin, according to Jafarzadeh. It comprises about 30 square kilometers in a fenced area, he said.

Using a series of slides, Jafarzadeh said satellite imagery of the site showed no activity in 2004, extensive excavation work in 2008, and more progress in 2009. Now there are 2010 images "confirming" nuclear activity, he said. He pointed out four tunnels, and said sources inside Iran told his organization that these tunnels access underground facilities intended to hold gas centrifuges.

Jafarzadeh said the findings were recently presented to the U.S. government.

A U.S. official said the intelligence community has known about the construction of the new facility for years, but "there is no reason at this point to think the facility is nuclear."

Although there is "still some ambiguity" about the facility's "ultimate purpose," the official said the intelligences pictures of this facility "are different" from those taken during the construction of the secret uranium enrichment site near Qom, which was first made public a year ago. The official suggested the site could be for "more conventional military purposes. ... The Iranians put military stuff in tunnels, too."

However the official did say Iran could modify the site in the future to make it a nuclear facility.

Some nuclear experts also are skeptical.

The Institute for Science and International Security issued a statement saying it cannot be determined whether this tunnel facility is a uranium enrichment plant.

"There is also reason to be skeptical of NCRI's claims, since so many of their assertions about secret sites have turned out to be unsubstantiated, exaggerated or wrong," the institute's statement said, but the International Atomic Energy Agency "would be more than justified in asking to visit a range of tunnel facilities in Iran, perhaps including the one identified by the Iranian opposition group."

The Iranian government says the site is an extension of the nearby Ministry of Defense training garrison called Javad-nia, according to Jafarzadeh. The regime has named the newer site Javad-nia 2 "to keep the site secret from local inhabitants," he said.

Referring to a November 2009 International Atomic Energy Agency report in which Iran denied any nuclear facilities were under construction, and said any such future facilities would be disclosed, Jafarzadah said, "I think the Iranian officials were lying through their teeth as they made those statements to the IAEA, as they have been for years."

Soona Samsami, who like Jafarzadeh speaks on behalf of the PMOI in the United States, said at Thursday's news conference that the facility is about 85 percent complete and is about a year away from "realizing the intended nuclear objectives." She would not comment more specifically on what those objectives might be, or what exactly the facility would be capable of at that time.

Samsami said the Iranian regime has so far spent $100 million on the project, and called it a new part of the regime's command center for nuclear weapons manufacturing.

The PMOI advocates the overthrow of the Islamic Republic of Iran, and has called for its removal from the State Department's terrorist organization list.

Iran has so far rebuffed international demands that it halt its uranium enrichment program, insisting it wants the nuclear fuel for peaceful uses. In low concentrations, enriched uranium can be used to fuel civilian power plants -- but in extremely high concentrations, it can be used to produce a nuclear bomb.

The Iranian government announced last year that it intends to build another 10 nuclear uranium enrichment plants for energy purposes.

The United States has accused Iran of working toward nuclear weapons, while the International Atomic Energy Agency has called for increased safeguards at the Iranian enrichment plant at Natanz to ensure it is not concealing nuclear material.

CNN's Pam Benson contributed to this report.