Washington - The US faces a growing threat of a "cyber-Pearl Harbour" and has drafted new rules for the military that would enable it to move aggressively against digital attacks, Defence Secretary Leon Panetta said.
The amended rules of engagement underline the need to defend Defence Department computer networks, "but also to be prepared to defend the nation and our national interests against an attack in or through cyberspace", he said.
Citing a mounting cyber danger that could cripple the country's vital infrastructure, Panetta told the audience in New York that "we won't succeed in preventing a cyber attack through improved defences alone".
"If we detect an imminent threat of attack that will cause significant physical destruction or kill American citizens, we need to have the option to take action to defend the nation when directed by the president," he said.
"For these kinds of scenarios, the department has developed the capability to conduct effective operations to counter threats to our national interests in cyberspace."
Although he avoided the word "offensive" to describe operations or capabilities, Panetta's speech clearly implied that the military would be empowered to take the initiative in the cyber realm.
Officials offered no further details, but as former CIA director, Panetta reportedly helped oversee an unprecedented cyber sabotage campaign that targeted Iran's uranium enrichment programme.
US President Barack Obama's administration has not publicly acknowledged the operation, dubbed "Olympic Games", which was detailed in a book by New York Times reporter David Sanger, based on interviews with officials.
"All of those who want to do us harm must know that the Department of Defence will take all action necessary to defend the nation," a senior defence official, who spoke on condition of anonymity, told reporters.
Panetta warned of a "significant escalation of the cyber threat", with foreign actors targeting "critical infrastructure networks", including systems that operate chemical, electricity and water plants, as well as transport.
He laid out dire scenarios in which hostile states or groups could seize control of vital networks.
The result could be "'cyber Pearl Harbour": An attack that would cause physical destruction and loss of life, paralyse and shock the nation, and create a profound new sense of vulnerability," he said.
"An aggressor nation or extremist group could gain control of critical switches and derail passenger trains, or trains loaded with lethal chemicals.
"They could contaminate the water supply in major cities, or shut down the power grid across large parts of the country," he said.
Panetta used the speech to press for cyber security legislation, arguing that major companies could not share information with the US government to thwart digital threats without legal protections.