Richard Clarke said the US faced a "very, very smart foe"

"America built cyberspace, and now it must defend cyberspace," Mr Clarke said in accepting his new position.

For years, he has been warning of what some refer to as Electronic Pearl Harbour - a computer-based attack that would cause massive amounts of destruction and loss of life.

In a worst-case scenario, such an attack would target power distribution, financial services, emergency call services and air-traffic control systems.

"We as a country are now dependent upon information technology and the networks that we have built," Mr Clarke said in an address last year to technology executives.

It is this dependence that leads some to believe that terrorists could target computer systems and wreak havoc on the United States. And the hijackers who took part in the attacks on 11 September are believed to have used the internet to communicate, often logging on at terminals in public libraries and copy shops to make their online activities difficult to track.

Officials say some hackers are trying to download military and national security secrets, and the US Department of Defense computer systems have suffered from many of the same viruses that have plagued corporate and personal computer systems.

Bombs still the main threat

Security expert Richard Forno said "Clarke is one of the few folks in the government who has a clue about the issue" of computer security.

It is much more effective to see a smoking crater where the World Trade Centre towers used to be than to see a darkened computer screen.

Computer security expert Richard Forno

Mr Forno developed the first information security program for the US House of Representatives and served as chief information security officer for Network Solutions. He is now Chief Technology Officer for the security firm ShadowLogic.

He is sceptical of the idea of such a dramatic kind of cyber-attack envisioned in Electronic Pearl Harbour scenarios. As many terrorism experts say, "bombs are better than bytes".

"It is much more effective to see a smoking crater where the World Trade Centre towers used to be than to see a darkened computer screen. Osama Bin Laden is not going to say, 'Allah be praised, we crashed the Nasdaq'," Mr Forno said.

My first reaction to the Office of Homeland Security is that it is a typical government reaction. It will create bureaucracy and jobs without necessarily being effective

Richard Forno

Cyber-attacks were more of a nuisance than viable terrorist tactics, he said. And he said he did not consider someone hacking Amazon or eBay to be an act of cyber-terrorism.

He also said that the government needed to respond to specific threats and not simply give in to knee-jerk reactions, such as the calls by some in Congress the day after the attacks to limit encryption.

The government also needed to assess what types of operating systems their critical systems used and assess the security of those systems, he added.

And to create effective information security policy, he said, there should be talks with chief technology officers and systems administrators who dealt with computer security on an ongoing basis.