The US Air Force has warned its troops that using Facebook and other social networking sites could inadvertently reveal their location to the enemy.
The warning, posted on an internal website and sent to commanders, concerns new technology allowing users to pinpoint their location on the map.
It said careless use could have "devastating operations security and privacy implications".
The US Army is set to send a similar warning to its troops.
Air Force officials are concerned that troops using BlackBerrys and other devices with GPS capabilities could reveal their location to internet services with geo-location features - Facebook, Foursquare and others - and that the enemy could discover their location, the Associated Press reported.
On most of those sites, users can adjust privacy settings to enable or disable the geo-location features.
The US has about 95,000 troops in Afghanistan and 50,000 in Iraq.