The United States central command was hit by an attack and affected computers in combat zones in the Middle East.
Military leaders briefed President Bush following suspicions that the attack may have originated in Russia, posing unusual concern among commanders and potential implications for national security.
U.S. central command is the headquarters that oversees U.S. involvement in Iraq and Afghanistan, though defence officials would not describe the extent of damage inflicted on military networks.
Military computers are regularly hit by hackers, computer viruses and worms, but Defence officials said the most recent attack involves an intrusive piece of malware that was apparently designed specifically to target military networks.
Electronics experts have not pinpointed the source or motive of the attack and could not say whether the destructive program was created by an individual hacker or whether it had Russian government sponsorship.
One defense official said the military has also not learned whether the software's designers might have been specifically targeting computers used by troops in Afghanistan and Iraq. Speaking anonymously, the official told the Baltimore Sun: “This one was significant; this one got our attention.”
Although officials are withholding details of the attack, the seriousness of the response underscores the increasing danger and potential significance of computer warfare, which defence experts say could one day be used by combatants to undermine even a militarily superior adversary.
The invasive software, known as agent.btz, has circulated among nongovernmental U.S. computers for months, but only recently has it affected the Pentagon's networks. It is not clear if the version responsible for the cyber intrusion of classified networks is the same as the one affecting other computer systems.
The malware is able to spread to any flash drive plugged into an infected computer. The risk of spreading the malware to other networks prompted the military to ban the flash drives.