CIA director George Tenet

Mr Tenet told the committee that information warfare had the potential to deliver a crippling blow to American national security unless strong measures were taken to counter it.

"It is clear that nations developing these programmes recognise the value of attacking a country's computer systems both on the battlefield and in the civilian arena," Tenet said.

He said the CIA had identified several countries that included information warfare as part of their military doctrines and it had discovered at least one instance where American computerised information systems were being actively targeted.

He gave no further details, but said there was reason to be concerned about technological development in Iran, Iraq and Libya.

A weapon for the weaker state

He said that while such countries could never hope to overcome the US in conventional warfare, they realised they could stand a better chance if they targetted computer systems.

"These countries recognise that cyber attacks...against civilian computer systems in the US represent the kind of asymmetric option they will need to 'level the playing field' during an armed crisis against the United States," he said.

He also cited remarks by officials in China and Russia pointing to the value of hacking into American computer systems in a hypothetical attack.

"An adversary wishing to destroy the United States only has to mess up the computer systems of its banks by high-tech means," Tenet quoted an article in China's official "People's Liberation Daily" as saying.

Growing fears of cyber war

A BBC correspondent in Washington says the United States has become increasingly concerned about the vulnerability of computer networks used both by the defence and intelligence communities and those which govern power grids and air traffic control systems.

President Clinton has already called for an interconnected computer security system to defend against such attacks in the United States, but it will take several years to develop.

Earlier this year, an international group of computer hackers broke into Pentagon computer systems and claimed to have stolen key software programs from NASA.