The US National Security Agency (NSA) chief on Thursday urged top computer security specialists to harden the nation's critical infrastructure against inevitable destructive cyber attacks.

"This is an important time," NSA and Cyber Command director Gen. Keith Alexander said during a presentation at a premier RSA Conference in San Francisco. "Most of the destructive tools being developed haven't been used; we need to use this window of opportunity to develop defenses."

Two days earlier, Deputy Secretary of Defense William Lynn gave a similar warning, saying the capability clearly exists for malicious software to cause real-world damage at power plants, water supplies and other vital points.

"Few weapons in the history of warfare, once created, have gone unused," Lynn said during a speech at RSA. "It is possible to imagine attacks on military networks or critical infrastructure-like our transportation system and energy sector-that cause severe economic damage, physical destruction, or even loss of life."

"I believe we have the talent to build cyber security that protects our civil liberties and privacy," he said. "We can and must do both."