The Federal Bureau of Investigation (FBI) has begun to investigate a wave of attacks on some of the internet's most popular commercial websites.

US Attorney General Janet Reno pledged to use the full resources of the government to stop the unprecedented series of attacks on the websites.

Ms Reno said that "these cyber-assaults have caused millions of Internet users to be denied services".

She said the motives of the vandals are not known "but they appear to be intended to interfere with and disrupt legitimate electronic commerce.

"We are committed to in every way possible to tracking those who are responsible," she said.

So far, there are no indications as to who could be behind the attacks or what the motive could be.

Organised campaign

On Monday, the world's most popular website, search engine Yahoo was hit. On Tuesday, Amazon and eBay suffered cyber attacks. Online brokerages E*Trade and Datek, as well as web publisher ZDNet, were the latest victims of the well-organised campaign.

All companies experienced sudden and sustained difficulties with their websites, blaming them on so-called "denial of service" attacks, where a flood of meaningless data create virtual traffic jams.

David Jarrell, director of the Federal Computer Incident Response Capability, said at least 100 computers in the US and abroad appeared to have been enlisted in the attacks, unbeknown to their operators.

"Determining the controlling computer is virtually impossible," he said, as the attacks bounced across networks.

It was possible that a lone hacker had launched the attack, he said. Such hackers are also called crackers because of the damage they do.

Ronald Dick, a cyber-security expert at the FBI, said the hackers could face up to 10 years in jail and a $250,000 fine.

'Clogged up'

E*Trade is the second largest online broker in the US, and Datek the fourth largest. Both companies insisted that the security of customer accounts had not been compromised.

E*Trade spokesman Patrick DiChiro said "access to the site was essentially clogged up".

Less than 20% of customers were unable to log on, he said. The attack lasted about an hour, beginning at 1300 GMT on Wednesday.

ZDNet's website, which covers technology news, shut down for two hours.

There has been speculation that the attacks could be a demonstration of strength, preparing the ground for a blackmail attempt.

The attack on buy.com, for example, appeared to have been timed to coincide with the company's initial public offering on the stock market.

The blitzing of eBay may have been planned to embarrass the firm as it unveiled a venture with the Walt Disney Company's go.com to develop sites to sell movie props and other memorabilia.

All the companies said the saboteurs did not gain access inside their computers or retrieve information about their customers.

However, the chaos highlights the vulnerability of even the best-protected websites and could shake public confidence in the booming global e-commerce economy.