The Department of Defence (DoD) has revealed that 13 sites, including YouTube, MySpace and Photobucket, slow down the military computer network.
It is also thought that the DoD has moved to protect information, after the recent announcement that the US military will censor the content of soldiers' electronic communication.
US Korea Forces general B.B Bell said in an army memo: "This recreational traffic impacts our official DoD network and bandwidth ability, while posing a significant operational security challenge."
Service men will be able to visit the sites on their own personal computers, but access in areas such as Iraq and Afghanistan is limited mainly to Defence Department machines.
Other sites affected by the ban include Metacafe, IFilm, StupidVideos and FileCabi, along with social networking site BlackPlanet and music sites Pandora, MTV, 1.fm and live365.
The Pentagon insists the ban is based on technical reasons, but is being criticised for preventing soldiers from communicating with family members.
Noah Shachtman, who runs a national security blog for Wired, said: "This is as much an information war as it is bombs and bullets. And they are muzzling their best voices."
The DoD itself posted videos on YouTube earlier this year, showing soldiers in Iraq offering help to citizens and overcoming insurgents.
