Soldiers serving overseas will lose some of their online links to friends and loved ones back home under a Department of Defense policy that a high-ranking Army official said would take effect Monday.

The Defense Department will begin blocking access "worldwide" to YouTube, MySpace and 11 other popular Web sites on its computers and networks, according to a memo sent Friday by Gen. B.B. Bell, the U.S. Forces Korea commander.

If the restrictions are intended to prevent soldiers from giving or receiving bad news, they could also prevent them from providing positive reports from the field, said Noah Shachtman, who runs a national security blog for Wired magazine.

"This is as much an information war as it is bombs and bullets," he said. "And they are muzzling their best voices."

The sites covered by the ban are the video-sharing sites YouTube, Metacafe, IFilm, StupidVideos, and FileCabi, the social networking sites MySpace, BlackPlanet and Hi5, music sites Pandora, MTV, and 1.fm, and live365, and the photo-sharing site Photobucket.