The pictures taken on June 11 were obtained from DigitalGlobe, a commercial satellite imagery firm, by the Institute for Science and International Security, whose president is David Albright, a physicist and former United Nations weapons inspector in Iraq.
"The construction activity is taking place in the closest mountainous area to the Natanz site, strongly suggesting that the site is affiliated with Natanz,”said a written analysis by Mr. Albright and a colleague, Paul Brannan.
The researchers said the construction is new and was not evident in images taken in January by DigitalGlobe.
Tunnel entrances are not visible in the satellite photos, but two roads under construction can clearly be seen going up the mountain, with one appearing to lead into a tunnel facility, the analysis said.
The researchers noted that Iran had earlier built a tunnel complex near the Isfahan uranium conversion plant to protect a range of nuclear-related equipment and materials and natural uranium hexafluoride.
"Iran may be constructing a similar facility near Natanz, fearing that the underground halls at Natanz are vulnerable to destruction by military attack,”the researchers said.
"A tunnel facility inside a mountain would offer excellent protection from an aerial attack,”the researchers added.
They also said that the mountain site could be for the direct defense of Natanz, possibly with antiaircraft batteries or other weapons.