In a statement posted on the YouTube blog, the company said that it had taken down some of the videos identified by Lieberman's staff because they contained hate speech, gratuitous violence or in other ways violated community standards.

"Senator Lieberman stated his belief, in a letter sent today, that all videos mentioning or featuring these groups should be removed from YouTube -- even legal nonviolent or non-hate speech videos," the statement said. "While we respect and understand his views, YouTube encourages free speech and defends everyone's right to express unpopular points of view."

In his letter, sent Monday, Lieberman said that radical Islamist groups use YouTube to share recruiting and training videos.