The US military is blocking troops from using certain websites for sharing photos, video clips and messages.
A memo from General BB Bell, US Forces Korea commander, says use of YouTube, MySpace and 11 other popular sites via US military portals will be blocked.
The US says the use is taking up too much bandwidth and slows down the military's computer system.
But a US Strategic Command spokesman said a "secondary benefit" was to help operational security.
Personal computers
BBC defence and security correspondent Rob Watson says the decision could stop thousands of soldiers from communicating with friends and loved ones.
For many US soldiers serving overseas, YouTube and other similar websites are a popular way of keeping in touch, he says.
The spokesman for US Strategic Command and Joint Task Force-Global Network Operations said: "As these sites have become more and more popular, they've had an impact on bandwidth resources and network availability and we're having to restrict use of some of them."
He denied that the military was unhappy with the nature of content being posted.
"We're not stopping anybody looking at anything. It's not the nature of the stuff being posted."
However, he added: "Wherever you have sites this popular, you have the potential for malicious activity. A secondary benefit of restricting use of these sites is for operational security."
The block on accessing such websites will not affect those soldiers with their own personal computers. Those, though, are few and far between in places like Iraq and Afghanistan, our correspondent says.
The Pentagon only recently started posting its own videos on YouTube, showing soldiers in action in Iraq in a move designed to reach out to a younger audience and to show the successes of the US military.
In two months, the Multi-National Force-Iraq channel has climbed to 16th in YouTube's most subscribed-to listing and has, the military says, just passed the one million video-views mark.
The BBC's Laura Smith-Spark in Washington says the channel is also a belated attempt to counter the influence of Islamist extremist groups, that have used the internet to post footage of hostages or attacks on US forces.
