The National Council of Resistance of Iran (NCRI) said that work on the site in Sorkheh-Hessar, east of Tehran, is used by sections of the secretive Organization of Defensive Innovation and Research (SPND), overseen by the defence ministry.

A section devoted to geophysics known as the Chamran Group is at the site and, according to the NCRI, "works on projects related to underground nuclear tests".

Their focus has included tests "for preliminary explosions to build nuclear weapons and record results by seismometers". Previous tests had taken place at a site south of Semnan, the group said.

It alleged that Iran had "engaged in the secret and illicit purchase of military-grade sensitive seismometers from Russia" to carry out the work.

The NCRI did not provide proof that such testing work was taking place at the new site, but argued that its existence showed again that Iran was breaching the terms of the 2015 Joint Comprehensive Plan of Action (JCPOA) nuclear deal signed with world powers.

The United States walked out of the deal under President Donald Trump in 2018, but European signatories have sought to keep it alive, with analysts predicting a Joe Biden victory in the American elections could give it new life.

"Our revelation today once again proves the fact that the JCPOA did not prevent the mullahs' activities to acquire nuclear weapons and even the regime has reneged on its commitments stipulated in the JCPOA," the NCRI said.

Iran has always denied seeking a nuclear weapon, insisting that its atomic programme is aimed at producing energy. But Western powers have long suspected that the drive seeks to make nuclear weapons.

The NCRI is the political wing of the People's Mojahedin (MEK), a group which initially backed the 1979 Islamic Revolution that ousted the shah but rapidly fell out with the new authorities.

Its fighters took the side of Saddam Hussein in the Iraq-Iran war, and the group is now based in exile, with MEK activists housed in Albania. The NCRI benefits from significant influence in some quarters in Washington.

The group credits itself with announcing the discovery of the nuclear enrichment facility in Natanz in Iran in 2002, which first alerted the world to the existence of the nuclear drive.

Later media reports have indicated that the information was leaked by foreign intelligence services, though this has never been confirmed.

It said that the latest findings were "top secret intelligence" obtained by the MEK inside Iran.