These clips are shot and edited by Islamic militants and broadcast on YouTube, the world's largest video-sharing Web site. Videos shot by terrorist groups were previously posted on specialist Internet forums, where only those  who were looking could find. But like all things political Youtube has changed that.

Jeremy Curtin, a U.S. State Department official responsible for monitoring Internet propaganda, said authorities were aware of the footage on sites like YouTube but had not made any real headway in tackling the problem.

Like most video-sharing sites, YouTube's policy is to take down video only after receiving a complaint. The video can easily be posted and reposted before it is removed completely.

Experts believe advances in Internet technology will lead to a surge in well produced, homemade extremist videos. "It's practically impossible to stop these videos," said the State Department's Curtin. "You can close one channel and another one will open up."

Mark Rasch, a former Justice Department computer crimes prosecutor, said the videos at YouTube and other sites are evidence of "a new front in the propaganda battle."

"It's here to stay," Rasch said. "It's going to get worse - we are going to see real-time executions with higher production values."

Militant Islamic Groups Turn to YouTube