Mohammad Mohaddessin, an official of the Paris-based National Council of Resistance of Iran (NCRI), said in a statement issued in Brussels that the group had passed on the information to the International Atomic Energy Agency in Vienna and had asked it to send its inspectors to the site.
"Lack of firmness by the international community has offered the regime the opportunity to get closer to obtaining the atomic bomb," Mohaddessin said.
Iran insists its nuclear programme is destined for civilian rather than military means.
Mohaddessin also said the Iranian nuclear programme was being assisted by experts from North Korea.