The sudden flurry of digging seen in recent satellite photos of a mountainside in central Iran might have passed for ordinary road tunneling. But the site is the back yard of Iran's most ambitious and controversial nuclear facility, leading U.S. officials and independent experts to reach another conclusion: It appears to be the start of a major tunnel complex inside the mountain.

The question is, why? Worries have been stoked by the presence nearby of fortified buildings where uranium is being processed. Those structures in turn are now being connected by roads to Iran's nuclear site at Natanz, where the country recently started production of enriched uranium in defiance of international protests.

As a result, photos of the site are being studied by governments, intelligence agencies and nuclear experts, all asking the same question: Is Iran attempting to thwart future military strikes against its nuclear facility by placing key parts of it in underground bunkers?

The construction has raised concerns at the International Atomic Energy Agency, the Vienna-based U.N. watchdog that monitors Iran's nuclear program. On Friday, an IAEA spokeswoman confirmed that the agency has broached the subject with Iranian officials. "We have been in contact with the Iranian authorities about this, and we have received clarifications," said Melissa Fleming, the spokeswoman. She declined to elaborate.

Calls to Iran's U.N. mission in Vienna were not returned. IAEA officials plan to press the issue further in a previously scheduled visit to Tehran later this week, according to informed sources.

"The tunnel complex certainly appears to be related to Natanz," said David Albright, a former U.N. weapons inspector and president of the Institute for Science and International Security, a Washington-based nonprofit group that provided copies of the photos to The Washington Post. "We think it is probably for storage of nuclear items."

U.S. officials at several military and intelligence-gathering agencies said they are aware of the construction and are watching it closely, though none would comment publicly or speculate on the purpose of the tunnels.

A tunnel complex would reduce options for a preemptive military strike to knock out Iran's nuclear program, according to U.S. officials who closely follow Iran's nuclear activities. It also could further heighten tensions between the Bush administration and the government of President Mahmoud Ahmadinejad, who has said he is committed to pursuing a peaceful use of nuclear power.

In response to suggestions by Vice President Cheney and others that the United States might consider using force to halt Iran's nuclear ambitions, Ahmadinejad has shrouded the program in additional secrecy and threatened to suspend cooperation with international nuclear inspectors.

Iran has been enriching uranium at Natanz on a small scale for more than four years, creating a less-enriched product that can be used for generating electricity. With further enrichment, the uranium could be used in making weapons.

The commercial satellite photos, taken on June 11 by the firm DigitalGlobe, show two new roads leading to a construction site on the side of a mountain closest to the nuclear site's southern boundary. Although tunnel entrances are not directly visible, the photos show rocks and debris in large piles near the dig sites. There are no signs of construction in similar photos taken of the area six months ago.

In a report analyzing the photos, officials of the Institute for Science and International Security (ISIS) compared the new Natanz construction with a tunnel built by Iran inside a mountain near another key nuclear site. That site, located about 80 miles to the south and known as Esfahan, is home to a major nuclear research center and a factory that converts uranium to a form that can be enriched at Natanz.

Iran began the work at Esfahan quietly in 2004, digging a large, two-entrance mountain tunnel that it later acknowledged was meant for nuclear storage. Iran eventually allowed IAEA inspectors to visit the then-empty tunnel. Having separate underground bunkers near both sites would allow Iranian officials to rapidly evacuate sensitive materials to safe storage if an attack were believed to be imminent, Albright, the ISIS president, said.

The intended use of the Natanz tunnel cannot be ascertained from the photos. But "such a tunnel inside a mountain would offer excellent protection from an aerial attack," said the report by ISIS, which produces technical assessments of nuclear programs. "This new facility would be ideal for safely storing" natural and enriched uranium and the specialized equipment needed to make it, ISIS said.

A less likely possibility, according to the ISIS report, is that Iran might seek to use the tunnels to house centrifuges used in uranium enrichment. Iran's existing centrifuges at Natanz are in heavily fortified buildings built partly underground. Iran has acknowledged plans to expand its uranium enrichment, requiring tens of thousands of fast-spinning centrifuges.

In April, Iran unilaterally withdrew from an international treaty that would have required it to publicly disclose design plans for any new nuclear-related construction. The ISIS report said that Iran nonetheless "should disclose to the [IAEA] any activity in this area related to its efforts at the nearby Natanz site or another nuclear purpose."