Extremist groups long have used the Internet to distribute videos, audio recordings and other messages. But this is the first time they have tried to establish a presence on the microblogging site Twitter, which has proved a valuable organizing tool to the young activists behind the revolutions in the Arab world.

“Up until now, we haven’t seen the [extremist] groups themselves active in this space,” said William McCants, an analyst with the Center for Naval Analyses and founder of the blog Jihadica.com, which reports on extremist messaging.

“The question is: Is this the start of a trend, or is it an anomaly?” he said, adding that individual supporters of extremist groups appear to be using Twitter.

The Twitter feed @alemarahweb posts links to the official website of the Islamic Emirate of Afghanistan, as the Taliban calls itself. The feed, in Pashto - the language spoken in the Taliban heartland of southern Afghanistan and the tribal regions of Pakistan - has been active since December and has posted more than 670 tweets, as the 140-character Twitter messages are called.

Another Twitter account, @al_nukhba, was set up last month. It posts links to an Arabic-language website called Nukhba al-Ilam al-Jihadi, or Jihadi Media Elite (JME). The website features Arabic transcripts of audio and video messages from al Qaeda and its affiliates in Yemen, North Africa and Iraq, said Christopher Anzalone, who identified the feeds.

Mr. Anzalone, a doctorate student at the Institute of Islamic Studies at McGill University in Montreal who studies Muslim political movements’ use of multimedia, said the phenomenon is new.

“There are a small number of [extremist media] outfits with Twitter accounts,” he said, calling the groups’ use of the service “very nascent.”

Whether the groups continue to tweet, in part, “depends on how Twitter responds,” he said.

“It’s hard to tell who exactly is posting [the tweets],” Mr. Anzalone said, noting that the Taliban feeds only posted links to the group’s official site.

U.S. law prohibits the provision of services to designated terrorist groups, including al Qaeda and the Taliban. But establishing a link between an individual’s social media account and a designated terrorist group is often impossible to do.

Moreover, the First Amendment protects freedom of speech.

Yet the major social media sites all have terms of service that limit what can be posted by users, and Facebook and YouTube regularly remove material or close pages that promote or show terrorist violence.

Twitter did not respond to multiple requests for comment over two days.

Sen. Joe Lieberman, Connecticut independent and a leading advocate for censoring extremists’ use of social media, told The Washington Times that the company should take action.

“I would think [terrorist sympathizers using Twitter] would be violating the company’s terms of use, just as they violated the terms of use of Facebook and YouTube,” the senator said.

Mr. Lieberman pledged to “continue to monitor terrorist use of social media - and advocate for social media companies to enforce their terms of use - in an ongoing effort to help stem radicalization over the Internet.”

A staffer for Mr. Lieberman added that the senator’s concern is driven by the increasingly important role that social media played in helping terrorist groups find and radicalize recruits online.

In the past few years, observers say, the propaganda machine employed by al Qaeda and other extremist groups has become much flatter and more diverse. It has grown from 10 or so password-protected Internet chat forums to a wide range of websites - many of them publicly accessible and maintained by supporters with no off-line links to the real groups.

Mr. McCants noted that extremist groups traditionally had been “much more comfortable with the password-protected forums” that al Qaeda and associated groups pioneered several years ago.

“We’ll see how long this lasts,” he said of the Twitter feeds.

A spokeswoman for YouTube, who asked not to be named, told The Times that the website has community guidelines “that prohibit dangerous or illegal activities such as bomb-making, hate speech and incitement to commit violent acts.”

Facebook spokesman Simon Axton said: “There is no place on Facebook for people who promote violence, and we devote significant resources” to stopping misuse by terrorist sympathizers and others.

“We actively search the site for content that promotes violence, using sophisticated tools and lists from various sources,” he said.

In the past, Facebook executives have told The Times that those sources include U.S. government agencies such as the State and Treasury departments.