The site is being constructed some 15 kilometres (nine miles) east of the central-western city of Isfahan and "has been built to test centrifuges that enrich uranium," said Firouz Mahvi, spokesman in Austria of the National Council of Resistance of Iran.
Mahvi said the 150,000 hectare (370,500 acre) site known as Isfahan's "fuel research and production centre" included other installations and "120 to 180 centrifuges are planned to be installed at this facility."
"To ensure the continuation of its nuclear programme, even during international inspections or in case of aerial bombardment, the clerical regime has set up a collection of smaller nuclear sites in different parts of Iran," he added in a statement.
In August 2002, the armed opposition group revealed the existence of an alleged secret nuclear site in central Natanz where inspectors from the International Atomic Energy Agency (IAEA) discovered traces of enriched uranium.
The traces were found in centrifuges used to separate molecules of uranium 235 and molecules of uranium 238. Some 21 kilograms (44 pounds) of enriched uranium 235 is needed to make an atomic bomb, according to experts.
Iran, which has denied making nuclear weapons, said the traces were due to accidental contamination from imported nuclear material.
The IAEA has set an October 31 deadline for Iran to prove that it is not making nuclear weapons, and said Tuesday that it stood firm by that deadline.
IAEA spokeswoman Melissa Fleming said Tuesday that the body was listening to what the Iranian opposition had to say. "We're taking good note of these elements that have gotten to us," she said.
An Iranian official said in Tehran that Iran would open talks with the IAEA on Saturday. "Experts from the IAEA will be in Tehran from Saturday to start negotiations on the signature of the additional protocol," Iran's representative to the IAEA, Ali Akbar Salehi, told AFP Tuesday.
Iran's signature and implementation of the additional protocol to the nuclear Non-Proliferation Treaty (NPT) is a key demand of an IAEA resolution passed on September 12.
IAEA chief Mohamed ElBaradei is due in Iran on Thursday.