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State officials also denied that the affected emails contained any classified information relating to military operations.

"Elements of the... unclassified email system were taken offline yesterday afternoon due to a detected penetration," <

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said US Defence Secretary Robert Gates (pictured, left). However, he expected the system to be back online "very soon".

The Pentagon is an obvious target for hackers - according to Gates, it sees hundreds of attacks a day - but the cost of detection can be high. For example, there is Gary McKinnon, a UK computer expert, accused by Washington of the 'biggest military hack of all time'. He lost his extradition appeal to the States earlier this year and is facing 70 years in jail. Although he has admitted gaining access to US government computers, he denies causing any damage.

As for this latest incident, Gates himself was apparently unaffected, apparently he doesn't "do email". "I'm a low-tech person," he declared.