A hacker has managed to penetrate one of the Pentagon's e-mail systems, leading officials to take up to 1,500 accounts offline.
The e-mail system did not contain classified information relating to military operations, a spokesman said.
The Pentagon says it rebuffs hundreds of attempted attacks on its computer systems each day.
US Defence Secretary Robert Gates said that a "variety of precautionary measures" were being taken.
"Elements of the... unclassified email system were taken offline yesterday afternoon due to a detected penetration," Mr Gates said.
It just goes to show that no matter what level of security measures you have in place and how much you spend on them, nothing can ever protect you on the internet. BBC News Report