And behind the scenes, Silicon Valley firms are under increasing pressure to wipe extremist content from websites as Western intelligence agencies explore new technologies to identify returning fighters at the border.
Governments from France to Indonesia have launched urgent drives to cut off one of the Islamic State group's biggest sources of strength: foreign fighters. At the heart of the drive is mounting concern that the organization is training the next generation of international terrorists.
Those fears have gained urgency from the group's horrific methods: A British militant is suspected of beheading two American journalists, and a Frenchman who fought with the Islamic State group is accused in a deadly attack on a Jewish museum in Belgium.
With each video that ricochets around social networks, the militants gain new recruits.
"If neglected, I am certain that after a month they will reach Europe and, after another month, America," Saudi King Abdullah said Friday, calling for a strong international response to the onslaught in Syria and Iraq.
Britain has taken a particularly active role in censoring content deemed to break the country's strict rules against extremist propaganda. U.K. officials recently revealed they have been granted "super flagger" status on sites such as YouTube, meaning their requests to remove videos with grisly content or that encourage terrorism are fast-tracked.
Over the past four years, an Internet-focused counter-terror unit of London's Metropolitan Police instigated the removal of 45,000 pieces of content, the force said last week. Islamic State militants, however, have just as quickly found other, more decentralized platforms.
In the United States, officials are trying to identify potential jihadis by comparing travel patterns with those of people who have already joined the fight, a counterterrorism official said, speaking only on condition of anonymity because he was not authorized to discuss intelligence matters.
A French law to seize passports is being fast-tracked through parliament, and the government is ramping up arrests of increasingly young teenagers making plans for jihad.
That can mean last-minute arrests at the airport, as happened to a 16-year-old girl and her alleged recruiter trying to pass through security in Nice on Saturday, and to a man at Australia's Melbourne Airport who was pulled off a flight last week carrying tens of thousands of dollars in cash and the Islamic State group's black-and-white flag in his luggage.
Britain proposed laws Monday to let police seize the passports of those suspected of having traveled abroad to fight, while the Netherlands is making it easier to strip people of Dutch nationality and go after Internet providers that spread propaganda.
In Bosnia, authorities carried out a major anti-terror sweep on Wednesday. They detained 16 people suspected of fighting in Syria and Iraq and recruiting Balkan men to join Islamic militants there.
Anti-jihadi efforts are being ramped up in traditionally Muslim countries as well: Indonesia is breaking up meetings of Islamic State supporters and seizing T-shirts and other items promoting the group, and Tunisia is shutting down mosques and suspected financiers.
For the radicals who have already reached Syria, the focus of European spy agencies is on trying to identify them when they return. That can mean scouring social media sites for photos of foreign fighters or electronic intercepts for hints of terrorist activity abroad.
Officials are considering the deployment of more advanced techniques like voice recognition to identify suspected jihadis at border control by matching their conversations to those heard on militants' videos, former U.K. counterterrorism chief Bob Quick told The Associated Press earlier this year.
There is huge interest, he said, in "being able to identify these people at the border."
The concern is that returning fighters will launch attacks at home. Australia draws on lessons from Afghanistan a decade ago, saying of the 25 citizens who returned to Australia after fighting against Western interests there, two-thirds became involved in terrorist activities back home. Some remain in prison.
"The Australians and their supporters who have joined terrorist groups in the Middle East are a serious and growing threat to our security," Prime Minister Tony Abbott told Parliament on Monday. "People who kill without compunction in other countries are hardly likely to be law-abiding citizens should they return to Australia."
A compilation of government estimates shows more than 2,000 people with European passports have fought or are fighting in Syria and Iraq — with most looking to join the Islamic State group.
But measures taken so far to keep Europeans from leaving have had no noticeable effect, said Trond Hugubakken, spokesman for the Norwegian security service PST.
"Preventive talks close to their departure doesn't seem to have any effect," he said. "It has an effect on some individuals, but if that's permanent or just delaying their traveling is hard to tell."
More than 1,900 Tunisian jihadis have joined the Islamic State group, according to government figures and Abdellatif Hannachi, a researcher on Islamic movements.
Britain, home to an estimated 400 fighters in Syria and now Iraq, had been emphasizing outreach to universities, religious centers and prisons. On Monday, the government said it hoped to bolster laws to block returning fighters from re-entering the U.K. and strengthen monitoring if they do.
Those proposals, like those under consideration in France and the Netherlands, marked a hard-line shift.
Over the weekend, influential imams in Britain issued an unprecedented fatwa against Islamic State militants, calling the group "a heretical, extremist organization." The religious decree banned British Muslims from joining and said they have an obligation "to actively oppose its poisonous ideology."
There has been little outcry in France — which has sent Europe's largest contingent of jihadi fighters to Syria and Iraq, estimated at 900 — to plans to seize passports of would-be jihadis and make it easier to shut down websites. France came up with the approach last spring, as the numbers of young French Muslims leaving for Syria started rising.
Prosecutors say 329 people are under formal investigation, including a 14-year-old girl who now faces up to five years in prison for allegedly wanting to leave home and fight.
Germany, which counts at least 400 people who have gone to Iraq and Syria, is especially concerned.
"What attracts people is their high brutality, their radicalism, their strictness," Hans-Georg Maassen, head of Germany's domestic intelligence agency, said Sunday. "We have to assume ... that there may well be people who return and commit attacks."
Contributors include Danica Kirka in London; Elaine Ganley in Paris; Karl Ritter in Stockholm; Bouazza ben Bouazza in Tunis, Tunisia; Paul Schemm in Rabat, Morocco; Rod McGuirk in Canberra, Australia; Eileen Sullivan in Washington, and Chris Brummitt in Singapore.
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