From the Middle East, Asia and Europe, extremists have built an extensive internet library of sophisticated texts on the ideology that underpins violence against the West and other enemies, Western analysts and intelligence officials said on Saturday.
"It's a steady, stealthy indoctrination aimed at creating a whole new generation of jihadists. And scandalously, it is unopposed," said Stephen Ulph, who studies the Islamists' use of the internet for the Jamestown Foundation, a Washington think tank.
E-books and online pamphlets, with titles such as "39 Ways to Serve and Participate in Jihad," encourage the growth of home-grown militant cells across the world, including in such Western countries as Canada and Britain, the experts believe.
US intelligence is reluctant to mount an effective counteroffensive by recruiting Islamic experts from overseas to rebut and even ridicule Islamist authors, according to experts and US officials.
"Anything exposing the West as a supporter would destroy Islamic opposition to the jihadis," one intelligence official on condition of anonymity said.
"We are completely out of luck with the Muslim world, across the board."
Several agencies including the CIA, FBI and the office of US National Intelligence currently monitor the content of Islamist websites.
But the programme is hampered by stringent security standards that make it hard for intelligence agencies to employ the necessary experts from the Arab world.
"Even if we think we understand elements of the religion, we certainly don't understand elements of their cultural communications," the intelligence official said.
Others warned that US policy-makers could be making a fatal error by ignoring doctrinal online texts that lay bare the substance of a violent Islamist mind-set.
"In order to be able to fight something, you have first of all to understand what is going on. And I don't think that at this stage they understand it well enough to fight it," said Rita Katz, director of the Site Institute, which tracks and analyses international terrorism.
In a presentation this week, Ulph said doctrinal material accounts for 60 per cent of Islamist web content and most texts are in Arabic.
But, he said, many have begun to reappear in English and other European languages in an apparent attempt to appeal to Muslims living in the West.
One of the most popular is the 1,600-page treatise, "Call to Global Islamic Resistance," a comprehensive guide to militant life by Mustafa Setmariam Nasar, an al-Qaeda ideologue also known as Abu Musab al-Suri, who was captured in Pakistan a year ago.
The internet first became a center for al-Qaeda operational planning, training and fund-raising after the fall of the Taliban in Afghanistan forced Islamists to find new ways to spread their message.
But Ulph and others, including former intelligence officials, say the future of Islamist militancy depends on the more sophisticated doctrinal material, capable of guiding the life of the committed militant from childhood to martyrdom.
"The focus has been on how these guys use the internet for fund-raising and operations," said Jarret Brachman of the Combating Terrorism Centre.
"Only recently have we realized there are strategic implications."