In 2010, when the group said it had evidence of another new nuclear facility, west of the capital Tehran, U.S. officials said they had known about the site for years and had no reason to believe it was nuclear.
The latest allegation comes less than a month after the election of a relative moderate, Hassan Rouhani, as Iran`s new president boosted hopes for a resolution of the nuclear dispute with the West, and may be timed to cast doubt on any such optimism.
Iran says its nuclear program is entirely peaceful and rejects accusations by the United States and Israel that it is seeking the capability to make nuclear weapons.
But its refusal to curb sensitive nuclear activity, and its lack of full openness with the U.N. nuclear watchdog, have drawn tough Western sanctions and a threat of pre-emptive military strikes by Israel.
The NCRI said members of its affiliated People`s Mujahideen Organisation of Iran (PMOI) inside the country had "obtained reliable information on a new and completely secret site designated for (Iran`s) nuclear project".
The NCRI which seeks an end to Muslim clerical rule in Iran, is the political wing of the PMOI, which fought alongside Saddam Hussein`s forces in the Iran-Iraq war in the 1980s.
The NCRI said the site was located in a complex of tunnels beneath mountains 10 km (6 miles) east of the town of Damavand, itself about 50 km northeast of Tehran. Construction of the site`s first phase began in 2006 and was recently completed, it said.
The group released satellite photographs of what it said was the site. But the images did not appear to constitute hard evidence to support the assertion that it was a planned nuclear facility.
A spokesman for the dissidents said he could not say what sort of nuclear work would be conducted there, but that the companies and people involved showed it was a nuclear site. The group named officials it said were in charge of the project.
"The site consists of four tunnels and has been constructed by a group of engineering and construction companies associated with the engineering arms of the Ministry of Defence and the IRGC (Iran`s elite Revolutionary Guards force)," the NCRI said.
"Two of the tunnels are about 550 meters (600 yards) in length, and they have a total of six giant halls," its statement added.