Cyber attacks pose the greatest threat to the United States after nuclear war and weapons of mass destruction -- and they are increasingly hard to prevent, FBI experts said Tuesday.

Shawn Henry, assistant director of the FBI's cyber division, told a conference in New York that computer attacks pose the biggest risk "from a national security perspective, other than a weapon of mass destruction or a bomb in one of our major cities."

"Other than a nuclear device or some other type of destructive weapon, the threat to our infrastructure, the threat to our intelligence, the threat to our computer network is the most critical threat we face," he added.

US experts warn of "cybergeddon," in which an advanced economy -- where almost everything of importance is linked to or controlled by computers -- falls prey to hackers, with catastrophic results.

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Michael Balboni, deputy secretary for public safety in New York state, described "a huge threat out there" against everything from banking institutions to municipal water systems and dams.

Henry said terrorist groups aim for an online 9/11, "inflicting the same kind of damage on our country, on all our countries, on all our networks, as they did in 2001 by flying planes into buildings."

A web attack of that scale has not yet happened in the United States but computer hacking -- once something of a sport for brilliant delinquents -- is rapidly evolving around the world as a weapon of war.

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