The release comes as US diplomats are stepping up pressure on their European partners in trying to persuade the International Atomic Energy Agency to issue a tough ultimatum to Iran demanding an end to all activities related to enrichment of uranium.

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

The pictures, presented by the Institute for Science and International Security, show what appears like an large industrial complex hidden in a natural 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 weapons 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," wrote weapons experts David Albright and Corey Hinderstein in their commentary to the images.

One of the most suspicious sights, according to the experts, is what they see as a high explosive testing bunker that could be used for development of nuclear weapons.

"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 point to a picture showing a construction site located about 1.2 kilometers (less than a mile) from the maim buildings of the weapons complex.

The excavation, they say, may also 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.

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