The question of foreign fighters and the security of Turkey's border with Syria has been raised again following the IS suicide attack in Istanbul's historic Sultanahmet Square, which killed 10 German tourists.
Intelligence sources speaking on condition of anonymity said strict measures had been taken at the border to prevent and catch foreign fighters trying to enter Turkey from Syria or vice versa, but admitted it was almost impossible to totally prevent illegal crossing.
The same sources said 961 IS members had been caught in Turkey by security forces in 2015, without naming the 57 countries from which the foreign fighters had come as they did not want the countries' representation offices in Turkey to react negatively, as was previously the case.
In December 2015, Turkey released a list of 913 foreign jihadists caught in Turkey in the first 11 months on the same year with open numbers of which countries they had come from. Most of the embassies in Ankara whose citizens were listed are reported to have reacted against being so openly cited.
China tops list of foreign fighters
According to the list announced in December, China tops the list with 324 citizens caught in Turkey fighting for IS. Russia comes second as 99 foreign fighters with Russian passports were caught in Turkey, while the Palestinians made up the third country in the list with 83 fighters. There were also 63 fighters from Turkmenistan, 57 from Afghanistan, and 44 from Indonesia.
Fighters include Trinidad Tobago, Maldives citizens
The caught fighters included in the December list came from far afield, with six carrying South Korean passports, five from Australia, four from Trinidad Tobago and two from the Maldives.
There were also a number of fighters from western European countries, including 19 from Germany and the United Kingdom each and 18 from France.
While remaining tight-lipped about the overall 2015 list, sources said two U.S. citizens were among the 961 caught and fighters carrying Syrian passports were also included. Sources did not share information about the other two countries among the 57.
The most frequently used area for illegal border crossings from Turkey to Syria and vice versa is reported to be the Elbeyli district of Turkey's southern province of Kilis.
Most IS fighters attempting to cross into Syria through Turkey claimed in their testimonies that they were simply trying to look for their relatives.
Some of the fighters crossing into Turkey from the Syrian side of the border said they had fled IS oppression, while others said they had been given bomb training at IS camps in Syria and were planning to conduct attacks inside Turkey.