The dissident National Council of Resistance of Iran (NCRI) exposed Iran's uranium enrichment facility at Natanz and a heavy water facility at Arak in 2002. But analysts say it has a checkered track record and a clear political agenda.
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An accusation it made in July about a secret underground nuclear site under construction in Iran draw a cautious international response, with France merely saying it would look into it.
The Islamic Republic says its nuclear energy program is entirely peaceful and rejects US and Israeli accusations that it is seeking the capability to make nuclear weapons.
The NCRI's latest allegation comes just a few days before Iran and six major powers are to meet in Geneva to try to end years of deadlock in a dispute over the Islamic state's nuclear program.
The election of Hassan Rohani, a relative moderate, as new Iranian president has raised hopes of progress towards a negotiated settlement of the decade-old nuclear row.
The Paris-based NCRI, citing information from sources inside Iran, said a nuclear weaponisation research and planning center it called SPND was being moved to a large, secure site in a defense ministry complex in Tehran about 1.5 km (1 mile) away from its former location.
It said the center employed about 100 researchers, engineers and experts and handled small-scale experiments with radioactive material and was in charge of research into the weaponisation of nuclear weapons.
"There is a link between this transfer and the date of Geneva (talks) because the regime needed to avoid the risk of visits by (UN nuclear) inspectors," Mehdi Abrichamtchi, who compiled the report for the NCRI, told a news conference.
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