The release comes as US diplomats are stepping up pressure on their European partners in trying to persuade the International Atomic Energy Agency to refer the case to the UN Security Council where Iran will face stiff sanctions.

US officials insist the work at the military complex is used by Tehran as cover for a clandestine nuclear weapons program.

The pictures, presented by the Institute for Science and International Security along with ABC News, show a large industrial complex hidden in a warren of valleys and crevices created by a mountainous plateau in northern Iran.

A paved road snaking in between barren hills connects warehouse-like buildings and smaller installations.

But expert commentary accompanying the material suggests that the Parchin conventional weapons complex, located about 30 kilometers (18 miles) southeast of Tehran, could also be used for nuclear work.

"This site is a logical candidate for a nuclear weapons-related site, particularly one involved in researching and developing high explosive components for an implosion-type nuclear weapon," weapons experts David Albright and Corey Hinderstein commented on the images.

Their suspicion is directed toward what they see as a high explosive testing bunker set in a rocky hollow.

"The concern is that this bunker could be where Iran would test a full-scale mock-up of a nuclear explosive using natural or depleted uranium as a surrogate of a highly enriched uranium core," Albright and Hinderstein pointed out.

The experts also drew attention to a construction site located about 1.2 kilometers (less than a mile) from the main complex.

The excavation, they said, may involve tunneling into the side of the hill, which suggests preparation for some type of armaments testing.

"Some analysts have suggested that this site does involve tunneling associated with an underground full-scale nuclear weapon test," Albright and Hinderstein noted.

They also point to two igloo-shaped structures built near the main weapons plant that "appear designed to contain hazardous operations."

The experts admit the evidence provided by the photos is "ambiguous" and cannot be seen as definitive proof of Parchin's clandestine use as a nuclear weapons facility.

But it appears sufficient to warrant a request for a visit by IAEA inspectors of this site, they said.

The Central Intelligence Agency and the State Department declined to comment on the documents, however, one US government official said Iran's nuclear program has been a longstanding concern to the US administration.

Parchin has been known as Iran's leading small arms and ordnance manufacturing complex since before World War II.

Over decades, say military experts, the plant has expanded its high explosives operation and branched out into missile and solid propellant production.

It is now reported to have Scud missile assembly lines as well as a facility manufacturing Fajr-3 missiles with the use of North Korean technology.

"It would be the logical place for Iran to conduct weaponization work on an atomic bomb and the logical place for us to look for such work," John Pike, director of GlobalSecurity.org, a local military affair think tank, told ABC News.

The pictures can be viewed at www.isis-online.org.

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