WASHINGTON — Defense Department officials said they’ve seen a drop in nonessential Web traffic and a boost in the military network’s performance since they blocked the popular Web sites YouTube, MySpace and 11 others from work computers worldwide.

Rear Adm. Elizabeth Hight, vice director of the Defense Information Systems Agency, said although the sites had not caused any specific problems yet, officials were concerned that high traffic to those sites could eventually snarl all online communication.

“This is in no way a comment on the content or uses of those sites,” she said in a Thursday briefing with reporters. “This is to preserve bandwidth for operational missions and enhance network security.”

The block went into effect Monday. It affects all computers on the military network, but does not stop troops or families from accessing the sites from non-Defense computers, such as commercial Internet lines at home or certain Morale, Recreation and Welfare computers off the official system.

Officials from YouTube are already petitioning the Defense Department to have the block on their site lifted.

“The vast majority of videos on YouTube posted by soldiers, their families and friends are personal messages, original songs, tributes and video letters,” YouTube spokeswoman Julie Supan said.

“We can’t speak for another organization’s policies, but we certainly don’t want YouTube to be used to share sensitive security information or put anyone in harm’s way.”

Airman 1st Class George Moss, who was recently deployed to Iraq, said he visited MySpace daily until the block was put in place, but said he has been frustrated since Monday because he has not been able to find a computer with reliable access to the site.

“It gets me through the day because I can go online and watch some of my favorite videos,” he said in an e-mail. “I have friends on MySpace who, the only way I can contact them, is through it.”

Hight said officials looked into partial blocks of the sites — blocking the sites during high-traffic times, or just from certain locations — but ultimately decided those approaches would not work.

Other sites may be added to the blocked list.

“This directive does not prohibit any individual from posting to or accessing these Web sites from their personal or commercial resources,” she said. “Many of these sites have been blocked on official networks in Iraq and Afghanistan for two years … so this should have no impact on deployed forces.”

She also added that soldiers can use their Army Knowledge Online accounts to send e-mails, pictures, and video clips similar to the services offered by the blocked Web sites.

Vernon Bettencourt, the Army’s deputy chief information officer, said the AKO offerings are less of a drain on the networks, and dismissed concerns that some soldiers might prefer to use a nonmilitary service to keep in touch with their friends and families.

“Of course we are able to monitor it, but we don’t monitor the sites heavily,” he said.

Sites blocked under the new rules:

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