The US government has warned that it could take military action against any terrorists who launch attacks through the internet.
In a move that could send cruise missiles heading toward hackers' houses, a White House technology adviser says the US "reserves the right to respond in any way appropriate" to tackle the growing number of internet warriors.
Advisor Richard Clarke says Iran, Iraq, North Korea, China, Russia and other countries are already having people trained in internet warfare.
Speaking at a Senate Judiciary subcommittee hearing on cyber-terrorism, Clarke said the US could use covert action but military action was one of the tools available to the president.
Mr Clarke refused to say what level of cyber-attack might lead to a military strike. "That's the kind of ambiguity that we like to keep intentionally to create some deterrence," he said.
This is despite the fact that the US has not found a foreign government or terrorist group using internet warfare.
Clarke added: "It does not mean that it has not happened or will not happen. If I was a betting man, I'd bet that many of our key infrastructure systems already have been penetrated."
"There are lots of cases where there has been unauthorised intrusions but we have never been able to prove to our particular satisfaction that a particular government did it," Mr Clarke said.