As ISIL insurgents threaten the Turkish border from Syria, Turkey is struggling to staunch the flow of foreign fighters to the militant group.
Thousands of foreign fighters from countries including Turkey, Britain, parts of Europe and the United States are believed to have joined the militants, according to diplomats and Turkish officials.
The ISIL militants, who seized an air base in northeast Syria on Sunday as they surge northwards, are trying to secure control of the area bordering Turkey above the city of Raqqa, their major stronghold, in a bid to further ease the passage of foreign fighters and supplies, sources close to ISIL said.
Some of the foreign fighters in their midst reached Syria via Turkey, entering the region on flights to Istanbul or Turkey's Mediterranean resorts, their Western passports giving them cover among the millions of tourists arriving each month in one of the world's most visited countries.
From Turkey, crossing the 900 km (560-mile) frontier into northern Syria was long relatively straightforward, as the Turkish authorities maintained an open border policy in the early stages of the Syrian uprising to allow refugees out and support to the moderate Syrian opposition in.
The advance of ISIL has alarmed Ankara and its Western allies, forcing them to step up intelligence sharing and tighten security cooperation.
"Thousands of Europeans have entered Turkey en route to Syria, and a large number of them we believe have joined extremist groups," said one European diplomat in Ankara, describing Turkey as a "top security priority" for the EU.
"In recent months especially we've seen a real hardening in Turkey's attitude, a recognition that this is a potential threat to their national security and a desire to take more practical steps through intelligence channels, police channels," the diplomat said, declining to be named so as to speak more freely.
That cooperation includes tighter screening of passengers on flights into Turkey in collaboration with European Union member states, and the beefing up of border patrols on the frontier with Syria, the diplomat and other officials said.
Turkey already kept a "no-entry" list of thousands of people suspected of seeking to join "extremists in Syria" based on information from foreign intelligence agencies, a Turkish official said, and barred more than 4,000 people from entering the country last year alone as a result.
Only three of 13 border gates between Syria and Turkey were now fully open, the official said, with foreign nationals only allowed to pass through two of them. Close to 70 people were detained in Turkey last year on suspicion of links with extremist groups in Syria.
"Security measures were increased a while ago as a result of the latest developments ... The Turkish armed forces believe the current precautions are sufficient," a second senior government official told Reuters.
Syria's rebels at the time enjoyed Western backing despite concerns about radical militants in their ranks, with Washington providing non-lethal aid and European states including Britain and France pressuring the EU to allow its arms embargo to expire.
Turkey has repeatedly denied harbouring or arming militants or turning a blind eye to their presence. Officials say it designated ISIL's precursor a terrorist group as long ago as 2005 and that Prime Minister Tayyip Erdogan vowed "zero tolerance" for al Qaeda-linked groups last November.
But they recognise a growing threat to their own security, particularly with ISIL fighters still holding 49 hostages seized from the Turkish consulate in the Iraqi city of Mosul in June, including the consul general, special forces' soldiers, diplomats and children.
They also refer to video footage filmed in Raqqa and broadcast this month by Vice News in which an ISIL activist said the group would "liberate" Istanbul if Turkey did not reopen a dam on the Euphrates river, prompting a government minister to respond that it would not surrender to such threats.
Sources close to ISIL in Syria say the group wants to take control of the border crossing at Jarablus, northwest of Raqqa. Earlier this year, it pushed out rival militants from the village to try to do so, but the Turkish authorities closed the passage.
ISIL also controls the area around the tomb of Suleyman Shah, grandfather of the founder of the Ottoman Empire, in northern Syria. The group has destroyed several shrines and tombs sacred to Shi'ites and other sects, stirring fears in Turkey that their next target might be Suleyman Shah.
The tomb is sovereign Turkish territory under a treaty signed with France in 1921, when Syria was under French rule, and Ankara has said it will defend the mausoleum.
But Turkey, along with its Western allies, could also face the threat of militant attacks on its own soil.
"I think they're waking up to the severity of the situation, particularly as the internal threat is getting higher and higher," said a second European diplomat, adding coastal resorts popular with European holidaymakers could become a soft target.
"It's a danger for Turkey because if ISIL decide that Turkey is an enemy (and launch an attack) then Turkey becomes like Egypt ... That's the end of tourism," he said.
Turkey's experience with a range of security threats, from Kurdish militants who fought a three-decade insurgency in its southeast to leftist extremist groups behind urban bombings, has left it with a formidable domestic intelligence agency.
But officials in Ankara estimate there are foreigners from more than 80 nations fighting in Syria and Iraq and say it is unreasonable for Turkey to act as "lone gatekeeper", stopping individuals who have travelled freely from their countries of residence after being radicalised at home.
"I don't think anyone has to worry about capabilities, but it's the scale of the threat and the speed it's evolving that any country would struggle with," said the first European diplomat. "And Turkey finds itself right on the front line."
REUTERSLast Mod: 26 Ağustos 2014, 15:18