Foxnews.com, theweatherchannel.com, espn.com and ABCNEWS.com have suffered denial of service attacks which disrupted services to hundreds of thousands of internet users.

The attacks began on Thursday and caused intermittent outages and slowdowns on the sites before normal service was restored on Friday night.

The hacks appear to have been a 'syn' attack, where a hacker uses hundreds or even thousands of computers to bombard requests to a target server.

The requests can overwhelm the server in minutes, blocking out legitimate users and rendering a site completely or partly inaccessible.

Officials are trying to locate the source of the attacks but, at the time of going to press, had not been able to do so.

The affected sites have contacted the police and the Computer Emergency Response Team's Co-ordination Centre.

It is the second time that the world's largest news, information and ecommerce sites came under a sustained and co-ordinated denial of service attack.

A series of attacks in 2000, traced to a Canadian teenager, prompted the news organisations to step up security, but officials admitted at the time that there is no guarantee that all such attacks could be prevented in the future.