Telephone hotlines and schemes to strip suspected militants of their passports are among eye-catching strategies to deter European fighters from joining Islamic State militants in Syria and Iraq.
But a more effective means - intelligence - is being under-utilized, according to diplomats and officials in Turkey, the main gateway from Europe to the militant's self-proclaimed caliphate.
Intelligence on recruiting networks is patchy and spy agencies are sometimes reluctant to share information, these officials say.
Thousands of foreigners from more than 80 nations, including Turkey, Britain, parts of Europe, China and the United States, have already joined the ranks of Islamic State and other radical groups in Syria and Iraq.
Undetected among tourists
Many crossed via Turkey, flying into Istanbul or its Mediterranean resorts on Western passports, undetected among the millions of tourists arriving each month.
More are trying, despite tighter border controls.
Turkish authorities have drawn up a “no-entry” list of 6,000 people, some as young as 14, based largely on intelligence from Western agencies, and have deported more than 500 suspected of seeking to join the extremists this year alone.
But the list is far from comprehensive, with gaps from some countries seen as major recruiting grounds in parts of North Africa and the Gulf, and some of the information already out of date by the time the Turkish authorities receive it.
“Our partners say we don't know who's going until after they've gone. So it's also about radicalization, about what happens before these people leave their homes,” said a senior Turkish official, declining to be named so as to speak freely.
“You can't catch the mice individually. ... There are networks involved. We have to crush these networks.”
Reluctance to share data
Some intelligence agencies are reluctant to fully share information with their Turkish counterparts, partly because Turkey does not share the same data protection standards and partly for fear of compromising the work of undercover agents seeking to infiltrate jihadist networks, security experts say.
“Data protection is an obstacle ... and this issue has put it under the spotlight,” said one European diplomat. “But I think we're sharing more than we used to, lists of names and other data.”
Cooperation has been more complicated with other states such as Egypt, the Arab world's most populous nation with which Turkey no longer has full diplomatic relations, or Libya, whose government was forced to flee the capital Tripoli last month after it was taken over by militants.
“Turkish intelligence asked us for this information, but we don't deal directly with them these days,” said an Egyptian intelligence official when asked about cooperation with Turkey.
“But we do give this information to other countries, like America, and we are sure Turkey can get it from them,” he said.
There have nonetheless been notable successes.
A man thought to be one of the main recruiters of French jihadists for the Islamic State group was placed in the hands of judicial authorities in Paris this month after his arrest in Turkey in August, after what French Interior Minister Bernard Cazeneuve described as “excellent cooperation.”
Spain, Morocco, Belgium and Germany have also made headway in breaking down recruiting networks, officials say.
But Turkish officials, rebuffing suggestions they failed to adequately control their border, point to examples of Western intelligence failings to highlight the challenges they face.
On one occasion this year a passenger arrived from a European airport with cartridges and the detachable barrel of a Kalashnikov in his luggage, the senior official said. Another who had been deported once, later appeared again at Turkish immigration with a different passport.
In February, two German citizens who were being investigated for links to extremists by the German authorities were caught in the southern city of Gaziantep trying to travel to Munich with explosives, the official said.
The German authorities have not commented on the case.
Purges among Turkish police
A European diplomat meanwhile said that recent purges in the Turkish police force, including of officers in anti-terrorism and organized crime units, had been a set-back.
President Tayyip Erdogan has been locked in a battle with a former ally turned foe, Islamic cleric Fethullah Gulen, whose followers he accuses of using influence in the police and judiciary to plot against him.
Thousands of officers have been dismissed or reassigned.
“There's a problem because of the purges within the police. People you've been working with and built up trust with are replaced or just disappear. It's hard to get information. Those who replace them are often very cautious,” the diplomat said.
“I sometimes get the sense of distrust (within Turkish institutions). ... Important information doesn't always end up in the right place at the right time.”
Turkish officials deny a lack of coordination, saying the interior ministry issued instructions to all relevant agencies in April on exactly what measures needed to be implemented.
The United States is drawing up plans for military action in Syria against Islamic State fighters, but Turkey, one of its closest allies in the region and a member of the NATO military alliance, is reluctant to take a frontline role.
That was due in part to fears for the safety of 46 Turkish hostages held by the militants. The hostages were freed on Saturday in what Erdogan said was a covert operation by Turkish intelligence, but Ankara's stance is not immediately expected to shift.
Its main role will instead be stopping the number of foreign fighters among the militant ranks, currently estimated by Turkish officials at between 8,000-11,000, from rising and intercepting those who try to return home via Turkey.
Attacks in home countries feared
Western governments fear their nationals, radicalized in Iraq or Syria, may stage attacks at home, with Australia warning on Friday of attacks on politicians and government buildings and saying it foiled a plot to behead a member of the public.
“There's a growing awareness of this problem, much more so than a year ago. ... There's no lack of political goodwill,” said one European diplomat of Ankara's tighter border security.
From Turkey, crossing the 900-kilometer (560-mile) frontier into northern Syria was long relatively straightforward, as the Turkish authorities maintained an open border policy to allow refugees out and support to the Syrian opposition in.
That policy has drawn accusations, strongly denied by the Turkish government, that it has supported militant Islamists, inadvertently or otherwise, in its enthusiasm to help Syrian rebels topple President Bashar al-Assad.
Foreign Minister Mevlut Cavusoglu, dismissing such suggestions as “unfair,” used a press conference in Berlin on Thursday to urge better information sharing to stop jihadists reaching Turkey.
“We are very determined on this issue. We know that every terrorist that comes to this region, whether foreign fighters or fighters from this region, are threats to Turkey,” Cavusoglu said.
“But ... it's ideal if they're identified before leaving their home countries,” he said.