WASHINGTON, Dec 2 (Reuters) - Islamic extremists are being instructed on how to use the popular video-sharing site YouTube as a way to disseminate propaganda videos, a U.S.-based terrorism monitor said on Tuesday.

Militants are being encouraged to use the online site through postings on other Islamic forums on the Internet, according to the SITE Intelligence Group.

Last week, an extremist authored step-by-step instructions on posting video to YouTube, which he described as "one of the most famous and biggest international sites that publish sections of videos from all over the world."

The posting encourages readers to post scenes of Western forces coming under attack to, it says, "shame the Crusaders by publishing clips of videos showing their losses, which they hid for a long time."

Islamic extremists have long used the Internet as a tool to communicate with supporters and distribute propaganda but the latest posting specifically coaxes militants towards YouTube and touts it as a user-friendly tool.

"I say that the YouTube site is one of the easiest sites to record and upload the clips," the posting states, pointing readers to the software they might need to publish on the Internet.

"I ask you, by Allah, as soon as you read this subject, to start recording on YouTube, and to start cutting and uploading and posting clips on the jihadist, Islamic, and general forums," the posting states.

YouTube, a unit of Google Inc. <GOOG.O>, could not immediately be reached for comment on how it might respond to the types of postings described in the message.

The message author calls for a "YouTube Invasion" by militants and includes several screenshots showing step by step instructions on how to create a YouTube account and to upload material.

The video-sharing site has often been a chosen venue for users to post controversial clips and other material.

In March, Pakistani authorities ordered Internet service providers to block the website after it ran material deemed insulting to Islam.

(Reporting by Patrick Rucker; Editing by Eric Beech)

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