The US is stepping up its efforts to combat the recent spate of cyber attacks on prominent internet sites.

The defence department has said it will check nearly 10,000 of its computer networks to make sure none was used to cripple major websites.

A Pentagon spokesman said the investigation was cautionary and there was no indication that orders might have been planted in any defence computers from hackers to assault websites.

The attacks will also top the agenda when Bill Clinton meets the nation's top computer security experts next week.

Executives from companies involved in e-commerce and computer networks, leaders from industry associations and academic experts who study online security have been invited to the meeting.

National security adviser Sandy Berger, and the president's chief of staff, John Podesta, will also attend.

The White House said the meeting had been organised following the president's budget proposal for $2.03 billion to protect the country's most important computer systems from online attacks.

But David Leavy, a spokesman for the National Security Council, said: "Certainly what happened this week will be at the top of the agenda."

The FBI is also reported to be meeting US security software experts to devise a system to hit back at the cyber attackers.

The UK's Financial Times said federal agents were meeting officials at a Silicon Valley software group to discuss using a "Cybercop Zombie" software to find and wipe out rogue programmes.

No idea

The United States authorities still have no idea who was behind this week's wave of crippling attacks.

Deputy Attorney-General Eric Holder said law enforcement officials were working to track down the culprits who had struck regularly since Monday.

"At this point, I can't honestly say that we have a sense of what the motive is," Mr Holder told reporters at the Justice Department.

"We don't consider this to be a prank. These are people who are criminals and we will do all that we can to find them, to prosecute them and to put them in jail."

He added that the cost in terms of lost business and repairs could run to tens of millions of dollars.

But he said investigators were still unclear whether they were looking for one or more attackers, or indeed whether those responsible were based in the US or abroad.

The latest targets to fall prey to the attacks include the online brokerage firm E*Trade and a technology news site ZDNet, which were both closed for a number of hours on Wednesday.

On Monday, the world's most popular website, search engine Yahoo, was hit. On Tuesday, Amazon, eBay, CNN and Buy.com suffered cyber attacks.

US officials described the hacks as a "distributed denial of service" attack on US businesses.

Such an assault swamps a website with so many requests for pages that legitimate users cannot access material, the cyber equivalent of jamming a telephone switchboard by making hundreds of calls simultaneously.