Lt. Daniel Zimmerman, an infantry platoon leader in Iraq, puts a blog on the Internet every now and then ''to basically keep my friends and family up to date'' back home.

It just got tougher to do that for Zimmerman and a lot of other U.S. soldiers. No more using the military's computer system to socialize and trade videos on MySpace, YouTube and more than a dozen others Web sites, the Pentagon says.

Citing security concerns and technological limits, the Pentagon has cut off access to those sites for personnel using the Defense Department's computer network. The change limits use of the popular outlets for service members on the front lines, who regularly post videos and journals.

''I put my blog on there and my family reads it,'' said Zimmerman, 29, a platoon leader with B Company, 1st Battalion, 28th Infantry Regiment.

''It scares the crap out of them sometimes,'' he said.

''I keep it as vague as possible,'' he said. ''I'm pretty responsible about it. It's just basically to tell a little bit about my life over here'' he said.

He's regularly at a base where he doesn't have Defense Department access to the Internet, but he has used it when he goes to bigger bases. He'll have to rely on a private account all the time now.

Memos about the change went out in February, and it took effect last week. It does not affect the Internet cafes that soldiers in Iraq use that are not connected to the Defense Department's network. The cafe sites are run by a private vendor, FUBI (For US By Iraqis).

Also, the Pentagon said that many of the military computers on the front lines in Iraq that are on the department's network had previously blocked the YouTube and MySpace sites.

The ban also does not affect other sites, such as Yahoo, and does not prevent soldiers from sending messages and photos to their families by e-mail.

Internet use has become a troublesome issue for the military as it struggles to balance security concerns with privacy rights. As blogs and video-sharing become more common, the military has voiced increasing concern about service members revealing details about military operations or other information about equipment or procedures that will aid the enemy.

At the same time, service members have used the Web sites to chronicle their time in battle, posting videos and writing journals that provide a powerful, personal glimpse into their days at war.

''These actions were taken to enhance and increase network security and protect the use of the bandwidth,'' said Col. Gary Keck, a Pentagon spokesman.

The Pentagon said that use of the video sites in particular was putting a strain on the network, and also opening it to potential viruses or penetration by so-called ''phishing'' attacks in which scam artists try to steal sensitive data by mimicking legitimate Web sites.

''The U.S. Army's not going to pay the bill for you to get on MySpace and YouTube,'' said Maj. Bruce Mumford, of Chester, Neb., who is serving as the brigade communications officer for the 4th Brigade, 1st Infantry Division, in Iraq. ''Soldiers need to know what they can and cannot do, but we shouldn't be facilitating it.''

After the warnings of the shutdown went out, military members were allowed to seek waivers if the sites were necessary for their jobs. Often insurgent groups post videos, including ones of attacks or—in some high profile cases—of U.S. or coalition soldiers who have been captured or killed.

''I guess it's a good general policy,'' Zimmerman said about the ban on MySpace and YouTube.'' If people could be trusted not to break operational security, then they wouldn't need to have the policy.''

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.''

Among 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.