Diplomats attending an International Atomic Energy Agency meeting in Vienna say Iran is building a system of deep underground tunnels to conceal components of its nuclear development program.

Iran is said to be trying to make its nuclear facilities impervious to "bunker buster" bombs and other advanced weapons systems.

Reports about Iran's effort to "harden" its nuclear installations have been circulating during this week's, three-day IAEA meeting in the Austrian capital.

Iran admits it began construction on underground tunnels beneath the city of Isfahan in September, to "increase capacity, safety and security of nuclear material." Associated Press reports that some tunnels are nearly one kilometer below the surface and are made of reinforced concrete.

An American envoy to the IAEA has told the agency's board of governors that the construction project has undermined Tehran's promises to suspend its nuclear development work.