Washington (AFP) - An exiled Iranian opposition group Tuesday accused Tehran of running a "secret" uranium enrichment site close to Tehran, which it said violated ongoing talks with global powers on a nuclear deal.

"Despite the Iranian regime's claims that all of its enrichment activities are transparent ... it has in fact been engaged in research and development with advanced centrifuges at a secret nuclear site called Lavizan-3," said Alireza Jafarzadeh, deputy director of the National Council of Resistance of Iran (NCRI).

He said the site was hidden in a military base in the northeastern suburbs of Tehran.

He presented to reporters a series of satellite images drawn from Google Maps which he said backed "this intelligence from highly placed sources within the Iranian regime as well as those involved in the nuclear weapons projects."

The Lavizan-3 site was apparently constructed between 2004 and 2008 and has underground labs connected by a tunnel.

"Since 2008, the Iranian regime has secretly engaged in research and uranium enrichment with advanced... centrifuge machines at this site," Jafarzadeh said.

The group had shared its information with the US administration, he added.

The existence of the site was "a clear violation" of the nuclear Non-Proliferation Treaty as well as UN resolutions and an interim November 2013 deal struck with global powers gathered in the P5+1 group, he said.

Under the interim accord, Iran agreed not to allow "any new locations for enrichment" and to provide IAEA, the UN nuclear watchdog, all information about its nuclear facilities.

"It is absolutely senseless to continue the negotiations," added Jafarzadeh.

The NCRI is a political umbrella of five Iranian opposition groups, the largest of which is the People's Mujahedeen Organization of Iran, which was once banned in Europe and the United States as a terror group.

The People's Mujahedeen has long opposed the nuclear negotiations, and with the NCRI has made several important revelations of the existence of secret nuclear sites in Iran.

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The so-called P5+1 group of Britain, China, France, Russia, the United States and Germany is trying to strike an accord that would prevent Tehran from developing a nuclear bomb.

In return, the West would ease sanctions imposed on Tehran over its nuclear program, which Iran insists is purely civilian in nature.

A new March 31 deadline is looming for agreement on a political framework, after two previous dates for a comprehensive deal were missed.

"Despite the Iranian regime's claims of transparency, these nuclear activities, today's intelligence, makes clear it has been continuing to lie for more than a decade," added NCRI member Soona Samsami.