The e-mail system did not contain classified information

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."