The group described the Reina nightclub as a gathering point for Christians celebrating their “apostate holiday”.
It said the attack was revenge for Turkish military involvement in Syria.
“The apostate Turkish government should know that the blood of Muslims shed with airplanes and artillery fire will, with God’s permission, ignite a fire in their own land,” the ISIL declaration said.
ISIL has been blamed for at least half-a-dozen attacks on civilian targets in Turkey over the past 18 months.
However, other than assassinations, this is the first time it has directly claimed any one of them.
It made the statement on one of its Telegram channels, a method used after attacks elsewhere.
What the Turkish government says
In a statement hours after the shooting, President Recep Tayyip Erdogan said such attacks aimed to create chaos and destabilise the country.
Speaking at a news conference in Ankara, Deputy Prime Minister Numan Kurtulmus said it was clear Turkey’s military operations in Syria had annoyed terrorist groups and those behind them.
“This attack is a message to Turkey against all decisive operations across the border,” Kurtulmus said, adding that the offensive in Syria would continue until all threats to Turkey were removed.
Kurtulmus also told reporters the authorities are close to identifying the gunman, after gathering fingerprints and information on his basic appearance.
Eight other people have been detained.
Kurtulmus said the authorities are monitoring hundreds of social media accounts that allegedly “were determined to sow seeds of enmity” and that legal actions has been taken against 92 people.
Kurtulmus also described the nightclub attack, in the early hours of 2017, as a message from extremist organisations that they intended to continue to be a “scourge” against Turkey in the new year.
But the deputy prime minister went on to say that “with all our national capacity, we will bring them to their knees”.
NATO member Turkey is part of the US-led coalition against ISIL.
In August, Ankara launched an incursion into neighbouring Syria to drive the radical Sunni militants and Kurdish militia fighters away from its borders.
The lone gunman remains at large.
Turkish police have distributed a photograph of the alleged gunman.
According to the Hurriyet newspaper, the authorities think:
- The attacker may be from a central Asian nation
- He has links to ISIL
- He may be from the same cell responsible for the gun-and-bomb attack on Istanbul’s main airport in June in which 45 people died and many were wounded
The attack – how it happened
The attacker is believed to have taken a taxi from the southern Zeytinburnu district of Istanbul.
Around 600 people were inside the Reina nightclub when the gunman shot dead a police officer and a civilian at the door.
Media reports say he forced his way inside and opened fire with an automatic assault rifle as well as throwing two hand grenades.
Witnesses say he shouted “Allahu Akbar” (God is greatest).
Some escaped and jumped into the Bosphorus after the attacker opened fire at random just over an hour into the new year.
Witnesses described how he shot the wounded as they lay on the ground.
Six empty magazines were reportedly found at the scene. The attacker is estimated to have fired at least 180 bullets.
Security services on alert
Security services had been on alert across Europe for new year celebrations following an attack on a Christmas market in Berlin in which 12 people died.
An online message from a pro-ISIL group called for attacks by “lone wolves” on “celebrations, gatherings and clubs”. (Euronews)