The head of Iran's nuclear agency, Ali Akbar Salehi, said the country has no undeclared nuclear sites, according to a report Friday by the semiofficial Mehr news agency.

Iranian opposition group the National Council of Resistance of Iran (NCRI) said on Thursday in Washington that in 2005 Iran began building a new uranium enrichment site in Abyek, about 120 km northwest of Tehran.

"We have no such installation that enriches uranium and if they (opposition groups) are aware of such a development, they should tell us. We will thank them," the country's atomic chief Ali Akbar Salehi told the Mehr news agency.

"No such nuclear installation with a specific definition exists in Iran which has not been declared to the agency," he said of UN nuclear watchdog the International Atomic Energy Agency (IAEA).

Iran is currently enriching uranium in the central city of Natanz and is building another such facility at Fordo, southwest of the capital.

Salehi said that nuclear installations have specific purposes.

"Facilities which are used for medical and agriculture purposes are not considered nuclear. There are many such (non-nuclear) facilities in Iran," he said.

The Pentagon and the independent Institute for Science and International Security (ISIS), meanwhile, expressed skepticism over the latest opposition claims.

"I don't know if this site is one that they have discovered that our intelligence experts have not seen. I find that hard to believe, but we shall see," Pentagon spokesman Geoff Morrell said.

David Albright, the founder of ISIS, said "it's just another one of their results you can't verify. It doesn't hold up very well in our minds. There's nothing in the satellite images that suggest a centrifuge plant." He said the NCRI have made "a lot of mistakes" with past claims about Iranian nuclear activity and they also "exaggerate their accomplishments dramatically."

The NCRI told Washington reporters that the Abyek site is "controlled, run and operated" by Iran's Defense Ministry, and that Iranian authorities have so far spent $100 million on the site and completed about 85 percent of the construction.