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.
He said it was expected that the system would be online again very soon.
Few details were available about the exact nature of the attack.
A Pentagon spokesman said attacks could come from a variety of sources including: "recreational hackers, self-styled cyber-vigilantes, various groups with nationalistic and ideological agendas, trans-national actors and nation-states."
Officials would not say whether the hacker was able to read e-mails sent on the system.
Asked if his own e-mail account was affected, Mr Gates said: "I don't do e-mail. I'm a low-tech person."
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