Amazon's websites suffered further sporadic outages on Monday, having been knocked out for about two hours on Friday.

The company is remaining tight-lipped on what is causing the problems, so speculation is rife in the blogosphere.

One explanation, according to security experts, is that Amazon is suffering a series of denial-of-service attacks.

Amazon's US website was unavailable for over two hours on Friday, as was the Amazon-operated Internet Movie Database.

A blog posting by Supranamaya Ranjan, at web traffic monitoring firm Narus, claims that the cause is a "network-initiated attack".

Web-performance monitoring firm Keynote Systems said that yesterday's outage affected US and UK Amazon sites and lasted for about an hour on Monday evening UK time.

Keynote researchers suggested that the speed at which the sites fell over, and the 'service unavailable' error message they showed to browsers, indicated an internal systems failure rather than DoS attack.