An email-borne attack has caused the US Department of Defense to take 1,500 of its internal computers offline. 

Secretary of Defense Robert Gates told reporters that the attack occurred in an unclassified email service, but added that attacks on the Pentagon's systems are nothing new.

"The reality is that the Defence Department is constantly under attack," he said. "A variety of precautionary measures are being taken. We expect the system to be online again very soon."

The attack is currently under investigation. Day-to-day operations have not been affected by the attack, according to Gates, and no other systems were compromised.

Gates himself was not hit hard. "I don't do email. I'm a very low-tech person," he joked.

Breaking into Pentagon computer systems can have very serious consequences. UK hacker Gary McKinnon recently lost his fight against extradition to the US on seven felony counts.

McKinnon has always claimed that he broke into government systems while searching for evidence of extra-terrestrial life. If convicted, he could be imprisoned for up to 70 years.