"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.
A statement from the Pentagon said the affected systems were quickly isolated and there were no reports of any impact on military operations.
On Friday new versions of the devastating computer virus threatened to wreak fresh havoc on computers across the world.
The computer security firm Symantec says it has found 10 copycat viruses able to elude software designed to block messages with the original Love Bug virus.
Computer analysts say that the damage caused by the virus could run into billions of dollars.
Pentagon infected
In a statement, Pentagon spokesman Kenneth Bacon said four classified internal systems were infected.
"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.
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.
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.