The latest attack took place on Sunday and is thought to have been tampered with for as long as nine hours before anyone noticed.

Army officials say the altered code was repaired early on Monday and insist that the army's sensitive internal network was not compromised.

"There were no security breaches," said Army spokesman Jim Stueve.

Other targets

Earlier targets have focused on sites run by the White House, the US Senate and the FBI.

Early on Tuesday hackers were also reported to have vandalised the US National Oceanic and Atmospheric Administration's Internet site for its Storm Prediction Center.

The center's director, Joe Schaefer, said the attack prevented emergency management officials across the US from using the site to check storm and tornado forecasts.

The weather service attack was first reported by the Attrition Internet service, which tracks hacker activities.

Settling rumours

The hackers behind the penetration of the Army site said their attack was "to settle rumours" about the demise of the hacker group that claimed responsibility for a similar attack on the White House site last month.

The electronic break-in apparently took advantage of a well-publicised flaw in the commercial software used to build the Army site.

The Army has said it is investigating the attack but gave no further details.

Homes raided

Recently the FBI has raided dozens of homes and seized thousands of dollars worth of equipment following threats from loosely organised hacker groups to vandalise every federal government website.

One such group, known as Global Hell, penetrated the White House site in May, leaving its GH logo splashed across the homepage.

The attack forced the White House site to close for more than 24 hours whilst programmers cleaned up the damage.