Unidentified hackers have penetrated US military computers in a widespread, two-week attack.

According to the Defence Department, the attacks have affected all the military services.

"It was the most organised and systematic attack the Pentagon has seen to date," said the Deputy Secretary of Defence, John Hamre.

Mr Hamre said the event had "all the appearances of a game" and was apparently perpetrated by "a small number of individuals".

He said that it did not appear to be connected with the ongoing crisis with Iraq.

The Pentagon official said the attacks appear to be directed at unclassified information, such as personnel records or payroll matters.

A possible explanation for the attacks is that they are part of a "hacker contest". But Mr Hamre did not offer any explanation about the origins or purpose of this contest.

"I think it was, in many ways, a wake-up call," he said.

He said he could not divulge too many details about the attacks because the military is working in co-operation with the Justice Department in trying to trace the perpetrators.

The Defence Department has been trying in recent years to update its defences against such attacks, but Mr Hamre said more must be done.

"We have to do a good deal more in this area," he said.

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