In a statement, Pentagon spokesman Kenneth Bacon said four classified internal systems were infected.

The "Love Bug": Tempting to open

"Despite these episodes, the Joint Task Force on Computer Network Defence says that it has received no reports that the virus had an impact on military operations," Mr Bacon said.

It is unclear how the virus penetrated the Defence Department's classified computers, which are physically separate from unclassified systems.

The classified systems use their own fibre-optic lines and computer terminals, designed to prevent intrusions by hackers or viruses.

Copycat viruses

One of the new copycat viruses entices users to open an e-mail with the word "Joke" in the subject line that unleashes yet another virus masquerading as a "Very Funny" attachment.

The Love Bug - so-called because it is contained in innocent-looking electronic-mail messages entitled "I love you" - is thought to be the fastest-moving and most widespread virus ever known.

Tackling the virus

Do not open it Delete it using shift del As with all e-mails, if in doubt do not run any attachments you are not expecting If you have run the attachment, isolate your machine from any network and phone your help desk or seek expert advice Remember to keep your anti-virus software up-to-date and be vigilant about attachments

The search for its creator, involving the FBI and Interpol, is focusing on the Philippines after a Manila internet services provider reported that the virus appeared to have spread from two of its e-mail addresses.

The bug has affected tens of millions of computers, according to some estimates.

California-based IT consultancy Computer Economics estimated worldwide damage to be $2.6bn by the end of Thursday. It said that figure could soar to $10bn by next week.

According to Eddy Hsia, of anti-virus manufacturer McAfee, the number of infected computers might jump this weekend as users log onto their home systems.

Dormant virus

Manila ISP Supernet said the original Love Bug e-mail was uploaded onto its servers as early as 28 April but remained dormant until Thursday.

The company said it suspected the author was a 23-year-old man living in Manila, who hid behind a smokescreen of hacked internet accounts, and pre-paid internet user cards.

However, it is possible that the creator was in another city or country.

The virus is believed to have caused billions of dollars of damage

Lithuanian officials said on Friday they would investigate whether the virus might have originated there, after a Love Bug began circulating with the title, in Lithuanian, of "Let's meet this evening for coffee".

Like last year's Melissa virus, the Love Bug is spread by e-mails that multiply once opened by the recipient, sending new messages to everyone in the user's address book.

The Love Bug is more destructive than Melissa as it overwrites audio and picture files, replacing them with its own code.