US OFFICIALS routinely refuse to talk about either computer defences or computer attacks America might have launched. But US offensive cyber retaliation could range from a passive intrusion such as listening in on a foe's communications to an attack that cripples an enemy's air defence systems to clear the way for a bomber attack.

A counterstrike on an attacker's computer network could be launched, Lewis said, but it would be extremely difficult.

'This is a gray area,' said Stewart Baker, who worked on cyber security at the Department of Homeland Security. 'But if you know that the North Koreans were doing this, then at a minimum I would have thought you'd be entitled to do the same thing to them to show that you didn't like it.'

If the attacks caused harm to anyone 'you get more serious, and start thinking and talking about it as an act of war or at least state-sponsored violence', said Michael O'Hanlon, a defence analysts at the Brookings Institution.

Though the recent computer attacks are considered by many cyber experts to be little more than a nuisance to public Web sites, the incident raised anew old criticism that the US government's policies on cyber warfare are shrouded in secrecy, ill-formed and require broad public debate.

'There's a lot of thinking that needs to be done about how to respond to attacks like this and what the threshold is for responding to cyber attacks, with other means, whether they be economic sanctions or even military force,' Ms Lord said.

The assault involved more than 100,000 'zombie' computers, used by someone without their owners' knowledge and linked together in a network known as a 'botnet.' Most of those computers were in South Korea, but others were in Japan, China, the US and possibly other countries.

'If you shoot back at the computers that actually launched the attack, then you're hitting third parties who probably don't even know they were involved,' Mr Lewis said.

'And if you go out over the networks to strike back at Pyongyang, how can you be sure you're not accidentally going to also take down Japan at the same time?' Said Mr Lewis: 'You could end up shooting the wrong guy.' -- AP