Iran is developing an underground military installation in the mountains west of Tehran, according to U.S. officials and Iranian dissidents, but the facility's exact purpose is in dispute, The Wall Street Journal reports.

An Iranian opposition group, the Mujahedin-e-Khalq, on Thursday told a Washington news conference that the site, which it called Javad-nia 2, is a nearly completed uranium-enrichment facility aimed at fast-tracking Iran's nuclear program.

A U.S. official disputed the MEK's finding that the construction site in western Iran is nuclear, and urged caution.

"This facility has been under construction for years, and we've known about it for years," said the U.S. official. "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 Iranians put military stuff in tunnels, too."

The MEK said the facility is 85% complete and adjoined to a major Iranian military garrison. The dissidents said they didn't believe that cascades of centrifuges, which are used to produce nuclear fuel, have been introduced to the mountainous site. But they said that three halls to house the centrifuges have been built and that the Iranian government has spent roughly $100 million developing the facility.

"This type of work has gone undetected and is expanding," said Soona Samsami, an MEK representative.

The MEK has played a significant role in recent years exposing some of Iran's covert nuclear activities. In 2002, it disclosed the existence of Iran's nuclear installations in the cities of Natanz and Isfahan. Subsequent investigations conducted by the United Nation's nuclear watchdog, the International Atomic Energy Agency, verified the MEK's claims and set off the international community's current standoff with Tehran over the nuclear program.

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