A large-scale assault by hackers has knocked many of Microsoft’s most popular websites offline. The attack comes hours after a supposed fault brought some of the same sites down for nearly 24 hours.

Sites affected for most of Thursday morning included Microsoft’s Hotmail free email service and MSN.com web portal.

Hackers targeted the routers that direct traffic to the company's websites. Microsoft said it took steps to defend against the attacks and that the sites were operating normally by Thursday afternoon. The company also said it had contacted the FBI.

Thursday's incident came just hours after Microsoft's websites were thrown offline for most of a day by what the company said was a technical error made by one of its engineers on Tuesday.

Microsoft claimed the incidents were not related, but Ric Steinberger, technical director at online security company SecurityPortal disagrees. “It seems strange for a company with the resources of Microsoft to take so long reaching a conclusion on the problem and implementing a fix. I think probably when the dust settles Microsoft is going to say this is all the result of a DOS attack."

There is, however, no evidence to support Steinberger's view.

Denial of service attacks disable a computer by bombarding it with a high volume of information requests in a short period of time, causing it to crash or become so overwhelmed it grinds to a halt.

Microsoft said it would take additional steps in the weeks ahead to protect itself further against interruptions. A spokesperson added: "It is unfortunate that an individual or group of individuals would engage in this kind of illegal activity.”