About 1,500 unclassified email users at the Pentagon had their service disrupted on Wednesday when a hacker infiltrated the email system, forcing the accounts to be taken offline.

In a briefing with reporters in Washington at the Pentagon, Secretary of Defense Robert M. Gates confirmed the incident and said that the users were disconnected from the system after the intrusion was discovered.

"The reality is that the Defense Department is constantly under attack," Gates said during the briefing. "Elements of the [Office of the Secretary of Defense] unclassified email system were taken offline yesterday afternoon, due to a detected penetration. A variety of precautionary measures are being taken. We expect the system to be online again very soon."

Gates said that his office "obviously [has] redundant systems in place, and there is no anticipated adverse impact on ongoing operations. There will be some administrative disruptions and personal inconveniences. It will come as no surprise that we aggressively monitor intrusions and have appropriate procedures to address events of this kind. But as I say, we get perhaps hundreds of attacks a day."

Gates said that he was not sure why the 1,500 users were removed temporarily from the system. "Well, I don't know the answer to that, and they're still investigating it." Gates said he doesn't use email, so he didn't know if his account was affected.

"I don't do email," he said. "I'm a very low-tech person."

A spokesman at the Department of Defense late this afternoon said he had no additional information about the incident.