PARIS (Reuters) - "I am French," explains the young man in the YouTube video carrying a Kalashnikov and wearing a kufiya cotton headdress as he sits in front of a waving black-and-white flag of al Qaeda.
"Oh my Muslim brothers in France, Europe and in the whole world, Jihad in Syria is obligatory," says the fair-skinned youth with sandy hair, wispy beard and southern French accent, imploring viewers to join him and his younger brother in Syria.
"There are many Muslims in the world and we need you."
Although the United States and its European allies support rebels fighting against Syrian President Bashar al-Assad, they consider some rebel groups to be dangerous terrorist organizations linked to al Qaeda.
Officials in Western countries say they are worried about the threat from their own nationals going abroad to fight in Syria and one day returning to carry out attacks at home.
"There is a key factor in the Syria war now: the number of French nationals who are fighting there. It is a problem of national security," a senior French diplomat told Reuters.
Radicals heading to Syria are learning about the war online from social media like Facebook, Twitter, YouTube and user forums. Security experts say that makes it harder than ever to disrupt the networks that might lure them in.
"The Islamist radicalization going on today isn't with preachers anymore, acting within mosques, but individuals who are using the Internet as a means of propaganda," said sociologist Samir Amghar, author of the book "Militant Islam in Europe."
As the West considers strikes on Syria to punish Assad's government for suspected chemical weapons attacks, as many as 600 Europeans have already joined the rebellion against him, according to the European Union, which in May recommended better tracking of social media to spot foreign fighters.
A much smaller number of Americans are also believed to be fighting. A Muslim convert from Michigan was the first U.S. woman believed to have been killed alongside the rebels in May.
Computer experts and police say online recruitment is particularly difficult to disrupt because of the dizzying volume of material, time lags in capturing digital evidence, the difficulty of cross-border cooperation and the uncertainty of securing convictions in countries that safeguard free speech.
"I describe it as a Sisyphean task," said Shiraz Maher of the International Centre for the Study of Radicalisation at King's College, London. "You try and pull it down and it will come back in one form or another."
"How do you begin to challenge this? It's just practically impossible to do, it's out there in such quantity."
Syria has now eclipsed conflicts in Afghanistan, Iraq, Libya and Mali to dominate web discussion by Islamists. Some 40 different rebel factions are uploading status reports from the ground in Syria in real time, said senior analyst Laith Alkhouri of security consultancy Flashpoint Global Partners.
Just a few keystrokes can uncover Germans, Italians, Belgians, British, Americans, even Australians - Muslim-born or recently converted - on social networking sites encouraging their countrymen to leave their homes and take up arms in Syria.
"Oh brothers! You don't need someone to take you by the hand to get there. A bit of resourcefulness and you're off!" wrote "Erwan" in a June 23 posting on French radical Islamist forum Ansar Al Haqq. He included links showing the easiest way to Syria from Turkey.
Authorities sometimes choose to shut or sabotage the sites of groups they identify as terrorists, as the United States and Britain did in corrupting online issues of al Qaeda's "Inspire" English-language magazine.
British Prime Minister David Cameron said in June that police had removed over 5,700 pieces of online terrorism content since 2011. Yet that is only a fraction of the estimated 50,000 extremist sites globally, according to the University of Arizona's Dark Web Project, which collects and analyses data from global jihadist forums.
While governments and major social networking sites quickly take down material deemed clearly offensive, such as videos of prisoners being beheaded, most content is less clear cut.
Authorities seeking to curb what they consider to be dangerous material on the Web have to make fine distinctions between political speech that is protected in most Western countries, and incitement to violence which is banned.
Sociologist Amghar said many of the sites are promoting an ideology, rather than calling for violence.
"The objective of many of these sites is not to incite individuals to commit attacks but rather to keep the idea of Jihad in the forefront of people's minds," he said. "The hard part to gauge with precision is what's the impact."
In a sign of the difficulty of stamping out extremism on the Internet, both France and Germany abandoned moves to block such content in the past two years.
The West's opposition to Assad muddies the issue further. It means any Westerners fighting against the government - and anyone on the internet urging them to do so - are ostensibly on the same side as Western authorities.
France's top anti-terrorism judge, Marc Trevidic, foresees challenges in prosecuting return Westerners who return home, given the difficulty of tracking their movements in Syria and proving they joined groups, such as the al Qaeda-linked al-Nusra Front, that Western states consider terrorists. A recruiting video may fall short of proof needed for conviction.
"We consider that wanting to fight Jihad is being a terrorist. But things aren't so simple," Trevidic told an anti-terrorism parliamentary committee in February.
France has opened five formal Syria-related terrorism investigations but no cases have yet been decided by a judge, according to a justice ministry source.
Across the Atlantic, a U.S. citizen, Eric Harroun, was indicted in June by a federal grand jury for allegedly fighting alongside the al-Nusra Front. He can be been seen in online videos posing with weapons and boasting of successful attacks.
"THANK YOU, YOUTUBE"
There are benefits to leaving extremist online material in place, security experts say.
"It's an excellent tool for intelligence," said criminologist Alain Bauer, a former security advisor to French ex-president Nicolas Sarkozy. "Western intelligence agencies should give Facebook, YouTube and these other sites a medal."
Eighty percent of terrorism cases before French courts rely exclusively on evidence from the Internet, according to a May 24 parliamentary report on terrorism.
"There's a sense of 'disrupt the flow' when they can, and also a sense of 'leave it be, let's monitor'," said Maher.
When authorities do try to take material off the Web, they are often too late to be effective. It may take months before YouTube responds to a government request to remove an offending video. In the interim, hundreds of copies may have been made and reposted, fuelled by buzz about the video on Twitter.
A system in which users flag inappropriate content is faster, but given that 72 hours of video is uploaded to YouTube every minute, it's not fast enough to keep such content offline.
"It takes five minutes to upload a one-hour video but it might take five months to get YouTube to be aware of this video," said Flashpoint's Alkhouri.
A spokesman for Google, which owns YouTube, said the company responds quickly after users flag content forbidden under policy guidelines, such as incitement to violence.
Given the flood of volume, Western police agencies need smarter tools allowing them to pinpoint and analyze the most dangerous content, a capability most don't have, said Hsinchun Chen, who runs Dark Web at the University of Arizona.
"The analogy is drinking water from the fire hydrant, the content just keeps coming through and how do you monitor that?"
Chen's Dark Web portal relies on multilingual data mining and content analysis to gather and sift through terrorist web content. He said a similar systematic method of collection is currently used only by Israel and one U.S. security agency.
"(Intelligence agencies) are experts in investigations but most of them are not experts in computer science. They don't have the resources or the will or the capability to collect large amounts of information on a systematic basis," Chen said. "They should have it, and it's available."
That also raises privacy issues, which have come to the fore in the United States since former spy agency contractor Edward Snowden revealed that the National Security Agency collects huge amounts of data from phone calls and internet traffic.
In its May report, France's parliamentary commission recommended strengthening the technical means and automation of Internet surveillance. It added that high-level engineers were "impossible to recruit."
URGENT OR LONG-TERM THREAT
Fighting online extremist content requires a cross-border response as websites may appear in one country but be hosted in another. But information-sharing can be slow and the sensitive nature of terrorism cases adds further delays.
"As soon as you talk about terrorism and national security there are other rules of the game," said Troels Oerting, head of the European Cybercrime Centre at Europol, which helps countries monitor the Web. "National security is very national, it's not very international."
One such example is Malika el Aroud, a Belgian-Moroccan convicted in 2007 by Switzerland for operating a website that recruited militant Islamist fighters to Afghanistan, only to launch a similar site across the border in Belgium. A Belgian court ultimately sentenced her to prison in 2010.
Police are likely to devote more effort to immediate local threats than hypothetical future threats, like those that might be posed by returned fighters from Syria.
"If I'm an intelligence officer in Paris and my primary concern is to make sure nothing happens on the Metro, I'm not immediately concerned by the guy saying, 'Go to Syria,'" said radicalization expert Maher.
"The urgent threat is the guy sitting in a Parisian suburb building a bomb," he said. "You have to balance resources between that threat and the important more slow-moving threat that will germinate and come to fruition in years to come."
(Additional Reporting By Mark Hosenball in Washington; Editing by Peter Graff)