Iran has been secretly building a vast underground nuclear plant 200 metres deep inside a desert mountain ridge in east of Tehran, in violation of the UN sanction regime, an anti-Iran group has claimed.

The People's Mujahedeen of Iran, which had previously revealed secret atomic plants at Natanz and Qom that the Iranian regime subsequently acknowledged, has said that it has already passed the information about the new "weapons programme" to the US government, The Telegraph reported on Friday.

The site, code-named 311, is set inside a military base near Abyek, 120 km outside the capital, and consists of a series of four bombproof tunnels made from reinforced concrete set 200 metres deep inside a desert ridge.

"This is certainly part of the secret weapons programme," said Alireza Jafarzadeh, a spokesman for the group who presented photographs of the site in Washington. "It's just moved underground, in tunnels, hidden from the outside world."

The exiled opposition group with an extensive network inside the country, said Tehran launched construction at the facility in 2005 and had spent $100 million on the tunnels.

The outfit is a radical but deeply rooted enemy of the Islamic Republic and its armed wing Mujahedeen e-Khalk is proscribed as a terrorist organisation in the US.

According to the British daily, the organisation has sent the information, including eyewitness reports from inside the facility and satellite images showing considerable development in the remote area, to the US government.

A Western diplomat said the UN's nuclear watchdog International Atomic Energy Agency (IAEA), would be expected to examine the report. If the information is credible the IAEA would demand access to the site to ensure that no refinement of uranium or other nuclear materials had taken place there.

"Any new plant would be in contravention of a six UN security council resolutions warning Iran to suspend all enrichment and reprocessing activities," the diplomat said.

Iran has maintained that it does not need to notify the IAEA of construction of a nuclear plant until 180 days before it is commissioned. However, the agency believes that Iran is under an obligation to confirm its intention to construct a plant.

The IAEA has established a substantial inspections regime at Natanz where, a report said this week, that Iran had processed 2.8 tonnes of low-enriched uranium, enough to manufacture two nuclear weapons, and 48lb of higher refined material that could more easily be converted into a bomb, the daily said.

According to the People's Mujahedeen of Iran, Tehran was believed to be about one year from commissioning the Abyek site but there has been a worrying escalation in the amount of electricity being consumed by the plant.

It said that electricity usage had doubled in recent months, though the group could not confirm that Iran had move centrifuge equipment to refine uranium to the plant.

Iranian officials have vowed to pursue a nuclear energy programme in defiance of international pressure. Officials have said there are plans to build 10 uranium enrichment plants in secret locations including mountain tunnels.