The US Department of Defense took an estimated 1,500 computers offline on Wednesday following a security breach within the Office of the Secretary of Defense (OSD).

"Elements of the OSD unclassified email system were taken offline yesterday afternoon, due to a detected penetration," said Secretary of Defense Robert Gates. "A variety of precautionary measures are being taken. We expect the system to be online again very soon."

Department of Defense officials had little more to say about the attack and would not confirm the number of systems that had been taken offline.

The Pentagon is exposed to "perhaps hundreds of attacks a day," and the department has back-up systems in place, Gates said.

"There is no anticipated adverse impact on ongoing operations," he added. "There will be some administrative disruptions and personal inconveniences."

However, it looks like the secretary himself will not be inconvenienced. "I don't do email," he said. "I'm a very low-tech person."