The US Marines have banned staff from using social media sites including Twitter.
The sites are considered a security risk, although marines whose work relies on access to them can apply to use them. This includes staff in press relations, criminal investigations and recruitment.
The US defence department is currently reviewing its policies on the use of social networking sites, and the chief information officer at the Pentagon has been asked to examine the options for dealing with the threats and benefits of the sites.
Bosses say social media creates too large a window for cyber attacks and makes leaking information too easy.
The approach is different to other parts of the military, which have embraced networking sites. The US Army made a well-publicised announcement that it wanted as many Twitter followers as actor Ashton Kutcher. Many defence organisations see the sites as a way of increasing support and communication, but they have to be carefully balanced with security concerns.