Satellite image of the Natanz Uranium Enrichment plant in Iran, taken June 11, shows the new tunnel facility inside a mountain near the key nuclear complex. Click here for additional detail. Reuters/Courtesy of DigitalGlobe-ISIS

"The construction activity is taking place in the closest mountainous area to the Natanz site, strongly suggesting that the site is affiliated with Natanz," the Institute for Science and International Security said.

The satellite imagery, taken by the U.S. firm DigitalGlobe, does not show the entrances to the tunnel. But the images show two roads under construction, one of which leads to the tunnel.

"Iran may be constructing a similar facility near Natanz, fearing that the underground halls at Natanz are vulnerable to destruction by military attack," the institute said. "A tunnel facility inside a mountain would offer excellent protection from an aerial attack."

The tunnel could also contain military assets such as air defense batteries to protect Natanz, the institute said. The report said the tunnel was not seen in images taken by DigitalGlobe in January 2007.

Iran has been building another tunnel near the nuclear research center at Isfahan. The project was said to have begun in 2004.

The International Atomic Energy Agency has been monitoring construction near Iranian nuclear sites. But IAEA director-general Mohammed El Baradei has asserted that Iran was slowing down uranium enrichment.

"We have seen a fairly slow development in commissioning new cascades," El Baradei said on July 9.