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WASHINGTON 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.

The policy is being implemented to protect information and reduce drag on the department's networks, according to Bell.

"This recreational traffic impacts our official DoD network and bandwidth ability, while posing a significant operational security challenge," the memo said.

The armed services have long barred members of the military from sharing information that could jeopardize their missions or safety, whether electronically or by other means.

The new policy is different because it creates a blanket ban on several sites used by military personnel to exchange messages, pictures, video and audio with family and friends.

Members of the military can still access the sites on their own computers and networks, but Defense Department computers and networks are the only ones available to many soldiers and sailors in Iraq and Afghanistan.

Iraqi insurgents or their supporters have been posting videos on YouTube at least since last fall, and the Army recently began posting videos on YouTube showing soldiers defeating insurgents and befriending Iraqis.

But the new rules mean many military personnel won't be able to watch those videos -- at least not on military computers.

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; social networking sites MySpace, BlackPlanet and Hi5; music sites Pandora, MTV, 1.fm and live365; and the photo-sharing site Photobucket.

Several companies have instituted similar bans, saying recreational sites drain productivity.