Soldiers' lives are being put at risk by geotagged photos - marked with a location - on social media sites like Facebook, the US Army has warned.
It says Facebook's new Timeline feature, which creates a map of places geotagged by users, also poses a risk to soldiers and their families
Many smartphones automatically geotag photos with GPS co-ordinates.
In 2007 four US Army helicopters were destroyed in Iraq after geotagged photos were posted on the internet.
By posting photos on Facebook or checking-in on social media sites like Foursquare or Gowalla, soldiers may reveal the exact location of their unit or their family, the US Army said in a statement. The release follows an army warning last year that geotag data in photos could tip off criminals.
"Do you really want everyone to know the exact location of your home or your children's school?" Staff Sgt Dale Sweetnam of the Online and Social Media Division said.