Military officials said they had restricted access to more than a dozen recreational sites because they had registered high levels of use on Department of Defense computers.

Rear Adm. Elizabeth Hight, deputy head of the Defense Information Systems Agency, said the Pentagon needed to ensure bandwidth on its network of more than 5 million computers was not clogged by the use of those sites.

"This network is critical for our effective and efficient and safe combat operations," Hight told reporters.

"We use it for everything from ordering supplies to sending orders to providing logistics information, scheduling people to get on an airplane, scheduling goods to move from point to point," she told reporters at the Pentagon.

Rep. Ed Markey, the chairman of the House of Representatives subcommittee on telecommunications and the Internet, has called on the Pentagon to reverse the decision, which took effect on Monday.

In a letter to Defense Secretary Robert Gates earlier this week, Markey said troops overseas had used many of the blocked sites to communicate with family and friends and that those contacts were critical for morale.

But the Pentagon said many of the sites had already been blocked on military computers in Iraq and Afghanistan for more than two years and troops had many other ways of keeping in touch with loved ones.

The Pentagon agency responsible for morale provided commercial Internet services free of charge at bases across Iraq and Afghanistan and those would be unaffected by the decision, Hight said.

She said the Pentagon had not banned troops from using the sites but had simply decided they could not be accessed from U.S. military computers to preserve bandwidth.

New technologies such as streaming video were real "bandwidth hogs," Hight said.

"We just simply cannot accommodate the growth in the bandwidth demands of this newer technology for both official reasons and recreational sites," she said.

The Pentagon said the blocked sites included YouTube, 1.fm, Pandora, MySpace, PhotoBucket, Live365, hi5, Metacafe, MTV, ifilm.com, Blackplanet, stupidvideos and filecabi.