Istanbul - Turkey risks encountering a new level of threat on its own territory from the Islamic State (ISIS) group after the Istanbul nightclub attack, with the group openly targeting the country as Ankara battles the jihadists inside Syria.While ISIS had been blamed for several attacks in Turkey over the last year, the brutal gun attack on the Reina nightclub 75 minutes into 2017 was distinctive in its choice of target and by the clear claim issued by the jihadists.Secular societyThe chairperson of the Centre for Economics and Foreign Policy and visiting scholar at Carnegie Europe, Sinan Ulgen, said: "The Islamic State has clearly decided to target Turkey."All the stages have been passed and battle commenced."The attack on the Reina nightclub that killed 39 people struck a symbol of the overwhelmingly Muslim country's secular society, where the Istanbul elite like to party, dance and drink.It took place in the district of Ortakoy, a traditionally mixed area which to this day has working churches and a synagogue.It also came on the night of New Year - as calculated by the Gregorian Calendar - which conservative Islamic hardliners in Turkey say should not be celebrated at all.Disputed claimOzgur Unluhisarcikli, Ankara office director of the German Marshall Fund of the United States, said the attack had sought "to drive another wedge between Turks with secular and conservative lifestyles."But the attack was also unique in that it was claimed by ISIS in a formal statement - the first time the jihadists have made a clear and undisputed boast of a major strike in Turkey, despite being blamed on several occasions.ISIS has in the past claimed individual assassinations of Syrian anti-jihadist activists in southern Turkey and there was a disputed claim over a bombing in the southeast in November."I think they're trying to make it look like something which is unambiguously centrally-directed, rather than that just carried out by a supporter," said Charlie Winter, senior fellow at the International Centre for the Study of Radicalisation and Political Violence at King's College, London.He said that over the last months, Turkey has been "firmly in the sights" of ISIS extremists "perhaps more than any other country", with threats repeatedly printed in its so-called magazine Dabiq.The attack came with Turkey deeply entrenched in a military campaign inside Syria aimed at ousting ISIS, as well as Kurdish militia, from the border area that has already lasted four months.In the early years of the Syrian civil war, Ankara's Western allies accused Turkey of turning a blind eye to or even abetting the rise of ISIS as a useful ally in the battle against President Bashar al-Assad.Heaviest casualtiesTurkey has vehemently denied such claims, noting it listed ISIS as a terror group since 2013. But the operation has transformed the equation in Ankara's approach to the group, making President Recep Tayyip Erdogan an arch-foe of the jihadists."The situation changed with the decision by Ankara to launch a military campaign against the Islamic State," said Ulgen.After a rapid start to Turkey's Syria operation, progress has stalled at the ISIS-held town of Al-Bab, where the Turkish army has sustained its heaviest casualties.Jihadists have been blamed for a slew of attacks in Turkey since 2015, including a devastating triple suicide bombing and gun attack at Istanbul's Ataturk airport in June that killed 47.But as well the jihadists, Turkey is also fighting Kurdish militants who claimed a double bombing in Istanbul after a soccer match hosted by top side Besiktas on December 10 that killed 44.