U.S. Marines can no longer access social networking sites on military computers.

The U.S. Marine Corps issued an order Monday banning sites like Facebook, MySpace, and Twitter citing a possible security risk.The order said using social sites could expose information to adversaries.

But the order doesn't effect Marines' private use of such networks on personal computers outside of their jobs.

The ban is part of a larger Pentagon review, which should be completed next month.

Defense Department spokesman Bryan Whitman said the department realizes that social networking sites have value, noting the Army recently ordered all U.S. bases to provide access to Facebook.

Military commanders in Iraq and Afghanistan have Facebook pages to share information about operations in the respective countries. For example, CNN found that Chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, has 4,000 followers on Twitter.

Whitman explained that a tension exists "between using these as important tools and addressing the challenges from a security standpoint." The goal of the review is to come up with "a more coherent policy," he said.

Sources: BusinessWeek, CNN