Internet users are being warned to watch out for a computer virus targeting popular social networking sites in the run up to Christmas.

Sky News has been told that Facebook users are most at risk from a variant of the Koobface virus, which has been disguised as a festive greetings video hosted on a YouTube site. Security experts say the new virus is "particularly nasty" and compels its victims to participate manually in creating a new Facebook account to help spread the worm.

In previous years, variations of this virus have caused chaos across the Web.

But Internet security experts believe the new worm is likely to prove particularly effective in fooling users, because of the growing number of people who now opt to send out electronic greetings cards.

The attacks work by posting malicious links on Facebook wall pages, inviting users to click on the Christmas card videos. However, any attempt to play the video turns over control of the PC to the attackers.

The victim then sees a warning message, requiring them to solve a Captcha (Completely Automated Public Turing Test To Tell Computers And Humans Apart) puzzle within three minutes. A timer ticks down and if the puzzle goes unsolved after the allotted time, the PC freezes up.

Experts warn that rebooting the computer will not help and the only way to end the loop is to solve the Captcha puzzle. The victim would then be able to use their PC as normal, but the attacker would still have control.

While those targeted grapple with the unfolding problem, the worm separately uses the victim's computer to fill out a new account application, unseen by them. The new account is then used to post more Christmas video worms in a continuing cycle.

Internet security company Panda Security has been tracking the latest variant of the Koobface worm from its labs in Spain.

"There is no doubting that this latest Koobface attack represents a serious threat to social networkers," the company's UK managing director Peter Lautin told Sky News.

"If someone runs the infected file on their Facebook or MySpace page, the worm will automatically log in to their account and several other social networking sites, sending malicious messages to all their friends.

"The more people who use an application such as Facebook, or any other means of social networking, the more likely they are to be targeted by bad guys to send out malicious threats such as Koobface."

The company recommends that users do not reply to or follow links included in unsolicited Facebook messages and users should always carefully check that the URL they are entering is really that of the site they want to access.