Hackers may try and trick targets into opening e-mail viruses

Over the weekend, the Honker group attacked US sites leaving pro-China messages on many and disabling others. The homepage of the Honker Union of China hosts a three-page list of all the sites it has claimed to have attacked.

Websites of Chinese news organisations said there had been public discussion of a "May Day war" in newsgroups in China. Many of the attacks that have occurred have been aimed at US Government organisations and six high-profile sites have been defaced, including the Department of Labor, Department of Health and Human Services, and the website of the US Surgeon General.

On some sites pictures of the dead Chinese pilot Wang Wei were used, and on others pro-China messages were left

'Denial of service' attacks

However, not all the attacks were well aimed. The White House Historical Society website was defaced which, despite its name, is not run by the US Government. Instead, it sells White House memorabilia.

Those hit by the attacks likened it to a vandal spraying graffiti on a shop window. Many of the sites were restored to normal working after a couple of hours. Others are yet to be restored.

Hacked US sites

Department of Labor Department of Health and Human Services U.S. House of Representatives US Surgeon General Department of Energy US Interior Department

But websites that watch hacking activity have been sceptical about the scale and significance of the attacks.

Many of the attacks have been dismissed as the actions of "script kiddies" who use software programs widely available on the net, rather than their deep knowledge of net software, to exploit well-known vulnerabilities in software.

But the FBI and the National Infrastructure Protection Center warned those looking after websites to be on their guard. Alerts issued by the organisations said there had been a "significant" rise in attempts to scan and probe websites.

The agencies speculate that this could be by hackers looking to place programs that can be activated to help with so-called "denial of service" attacks that flood sites with bogus messages making them unreachable.