Isis has claimed responsibility for the nightclub attack in Istanbul that killed at least 39 people.
The extremist militant group has released a statement after a gunman stormed the popular Reina nightclub on the banks of the Bosphorus.
A statement, reported by Isis-linked Aamaq News Agency, reads: "In continuation of the blessed operations that Islamic State is conducting against the protector of the cross, Turkey, a heroic soldier of the caliphate struck one of the most famous nightclubs where the Christians celebrate their apostate holiday."
It also said the man opened fire with an automatic rifle, believed to be a kalashnikov, in "revenge for God's religion and in response to the orders" of its so-called leader Abu Bakr al-Baghdadi.
Turkey President Recep Tayyip Erdogan has already said the country, which has lost hundreds of citizens to terrorism this year, will "fight to the end against terror".
The manhunt for the killer, who is believed to have fired up to 180 bullets in the 700-capacity venue, continues.
The onslaught, which murdered Saudi Arabians, Moroccans, Lebonese, and Libyans, according to a Turkish government minister, lasted approximately seven minutes.
Mr Erdogan earlier said: "As a nation, we will fight to the end against not just the armed attacks of terror groups and the forces behind them, but also against their economic, political and social attacks.
"They are trying to create chaos, demoralise our people, and destabilise our country with abominable attacks that target civilians.
"We will retain our cool-headedness as a nation, standing more closely together, and we will never give ground to such dirty games."
At least 69 people were being treated in hospital and four were said to be in a serious condition.
Security in Turkish cities had been heightened during the run-up to the New Year and 17,000 police officers, some reportedly camouflaged as Santa Claus, were deployed.
Twin bombings outside the Besiktas football stadium last month, an attack claimed by Kurdish militants, killed 38 people.
