With the recent spate of kidnappings and killings in Iraq and in Saudi Arabia, militants have made the internet their main channel of communication.
By Kevin Anderson
BBC News Online in Washington
They use it to communicate their demands and to distribute the grisly videos of the beheadings.
Video of the Nick Berg beheading was shown online
But beyond militants in the Middle East, extremist groups around the world have greatly expanded their online activities in the past decade, according to experts monitoring such activity.
The internet is used to spread propaganda, to find and profile potential recruitments, to raise funds, to research upcoming attacks and to communicate with one another.
And after al-Qaeda lost its training camps in Afghanistan, it has moved its training camps online.
Plethora of sites
"The internet is part of today's battlefield," said Brian Jenkins, a terrorism expert at the Rand Corporation think tank.
Militants have found the internet to be an extremely effective method of communication both to their members and to the wider world.
"The messages on the beheadings spread out worldwide in seconds," said Professor Gabriel Weimann of Haifa University, Israel, a communications professor who monitors the activities of terrorist organisations on the net.
And a recent study carried out in the US by the Pew Internet & American Life Project found that nearly a quarter of Americans online turned to the internet to view some of the graphic images of the war in Iraq that they could not find in mainstream media.
Mr Jenkins said that militant groups often complained in the past that mainstream media coverage of their attacks focused on the suffering of the victims and not on the group's political or ideological message.
With the internet, "they can ensure that the message goes out as they wish it to go out, with the images that they wish to convey," he told BBC News Online.
It is that effectiveness that has caused a rapid expansion in not only the number of sites maintained by such groups but also an increasing sophistication in the way they use the internet.
Web of terror
When Professor Weimann started tracking militant groups' use of the internet seven years ago, he found only 12 websites.
Now, he and a team of research assistants monitor 4,000 sites maintained by hundreds of these organisations around the world.
Hamas has several websites about its activities
Almost all major groups of this type have more than one form of presence of the internet, websites plus chatrooms, forums and bulletin boards, he said.
In fact, Professor Weimann said some groups, such as Hamas, have separate websites aimed at children.
And Hezbollah even created a downloadable video game called Special Force, based on battles between the group's fighters and Israeli forces in southern Lebanon.
"This is not an innocent computer game. It is training children to play the role of terrorist, even execute political leaders," said Professor Weimann.
The trend is not restricted to the Middle East but is global, including such groups as:
- The Peruvian Marxist group Tupac Amaru (MRTA)
- The Revolutionary Armed Forces of Colombia (Farc )
- Kahane, a Jewish group designated as a terrorist organisation by both the US and Israeli governments
- The Sri Lankan Tamil Tigers
- The Japanese doomsday cult Aum Shinrikyo, which carried out the sarin gas attack on the Tokyo subway in 1995
Al-Qaeda itself has devoted much of its attention to the internet.
"Since the war in Afghanistan, al-Qaeda moved to cyberspace, at least in terms of communication," Mr Weimann said.
Al-Qaeda is moving between some 50 different web addresses and has set up what they call online training camps, he said.
The net has been used by groups such as Farc
In January 2004, the group launched a spiritual magazine, called the Voice of Jihad, but they also began posting another magazine, al-Battar Training Camp.
Issue number 10 of al-Battar was devoted to kidnappings, and its release came just before the wave of kidnappings and executions, Professor Weimann said.
"If you look into the text, there are pages and pages on who should be kidnapped and when, how to negotiate, what should be asked for, how to tape the execution. It's all there," he said.