An Iranian dissident group claims it has discovered a newly completed secret underground nuclear site some 60 kilometers northeast of Tehran.

“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 Defense and the [Iranian Revolutionary Guards Corps],” the National Council of Resistance of Iran said in a statement released to Reuters.

It was unclear what specific nuclear work is carried out at the site, and satellite images released by the NCRI “did not appear to constitute hard evidence to support the assertion that it was a planned nuclear facility,” according to Reuters.

But the dissident group has been right more than once, and was the source that pinpointed both the uranium enrichment facility at Natanz and the heavy water facility at Arak which is nearing completion.

The new complex is reportedly located in tunnels under mountains some 10 kilometers east of Damavand, a town about 50 kilometers northeast of the capital Tehran. The site has been under construction since 2006, and was completed recently, according to NCRI.

“Two of the tunnels are about 550 meters in length, and they have a total of six giant halls,” according to NCRI’s statement.

The group is opposed to the theocratic regime that has ruled Iran since the Islamic revolution of 1979, and is affiliated with an armed branch, the People’s Mujahideen Organization of Iran, which it said had obtained the information about the new site.

Iran is currently under harsh UN sanctions targeting its energy and financial sectors as the regime is believed by Israel, the US, European states and others to be pursuing nuclear weapons capability. Tehran has remained defiant, stating several times in recent years that it planned to expand its nuclear program and construct new facilities throughout the country.