ISTANBUL -- Terror group ISIS has claimed responsibility for the deadly New Year's attack that left at least 39 club patrons and staff dead at an upscale Istanbul nightclub.
"In continuation of the blessed operations which ISIS carries out against Turkey, a soldier of the brave caliphate attacked one of the most popular nightclubs while Christians were celebrating their holiday," a statement posted to Twitter reads.
The claim, which comes a day and a half after the attack, cannot independently be verified by CNN -- ISIS did not provide sufficient evidence to prove that the attacker carried out the attack on its behalf.
It adds that "the apostate government of Turkey should know that the blood of Muslims who get killed by Turkey's planes and artillery will set afire inside their country."
The statement did not contain any information about the attacker.
The Kurdistan Worker's Party, or PKK, earlier issued a statement distancing itself from the attack, which took place during the early hours of January 1.
"No Kurdish forces have anything to do with this attack," the statement read. "The Kurdish freedom fight is also the fight for democratization of Turkey. That's why we won't target innocent and civilian people."
Turkish authorities continue to hunt for the man they believe is responsible for the bloody attack, which also saw 69 people hospitalized, including one American. A handful of the injured were in critical condition. As of Monday, 46 were still being treated, according to the Istanbul governor's office.
The victims hailed from 14 countries, according to Turkey's semi-official news agency Anadolu, including India, Morocco, Jordan, Canada, Russia, Tunisia, Saudi Arabia and Kuwait.
The suspected shooter, who stormed the Reina nightclub soon after the clock struck midnight, appears to have evaded the tight security that blanketed Turkey's largest city over the New Year. Yet, authorities are confident that they will apprehend him soon.
"There is strong coordination and we will find him, no delay," Turkish Prime Minister Binali Yildirim told reporters gathered outside an Istanbul hospital, where he had been visiting people injured in the attack.
Footage of the attacker shooting a security guard and police officer at the entrance of the nightclub has emerged, lending credence to Turkish authorities' claim that he carried out the attack alone.
A security official showed the entrance to CNN, confirming that it was the site of the video.
Despite no group yet claiming responsibility, Turkish authorities quickly characterized the attack as the work of terrorists.
"We are face to face with a terror attack," Interior Minister Suleyman Soylu told reporters Sunday morning, adding, "the efforts to locate the terrorist are ongoing. Security forces have begun the necessary operations. God willing, soon, (the attacker) will be apprehended."
World leaders condemned the shooting and US officials also called it a terrorist attack, making it the first of 2017.
'At first, we thought it was a fight'
Witnesses said a celebratory evening turned into a bloodbath.
"We were having fun. At first, we thought it was a fight, then there was a lot of gunfire," eyewitness Yunus Turk said after police secured the nightclub in the Ortakoy neighborhood.
"After the gunfire, everyone started to run toward the terrace. We ran as well. There was someone next to me who was shot and fell on the floor. We ran away and hid under the sofas."
Twenty-four men and 15 women, said Ali Seker, a parliament member, according to Anadolu. Of those, 27 were foreign nationals and 11 were Turks, Seker said. One person is still unidentified.
CNN Intelligence and security analyst Bob Baer said the attack bore the hallmarks of an attack perpetrated by, or at least inspired by ISIS.
"I think it's probably Islamic State, or (the gunman) is affiliated at the very least. (An attack targeting) celebrating New Year's is highly symbolic for the philosophy of the Islamic State," he said, using a different name for ISIS.
Around 1:15 a.m. Sunday, the gunman shot a police officer who was guarding the front gate at the nightclub, killing him, Istanbul Governor Vasip Sahin said.
"He entered the club and attacked innocent people who were there to celebrate the new year. It was a cruel, cold-blooded act," the governor said.
Witnesses in the club said they heard a loud noise, then a security officer told everyone to get out.
One victim said he didn't know how many attackers there were, but he saw one person and hid.
"I got shot in the (expletive) leg, man," he told journalists as he was taken into an ambulance. "These crazy people came in shooting everything."
The club issued a statement on its Facebook page Sunday.
"This terrible incident is a terror attack against our citizens' peace, brotherhood, serenity, economy, tourism and against our nation. Our hearts bleed and the bullets are in our heart," the statement read.
Attack on 'Western lifestyle'
Turkey has endured a recent wave of terrorist attacks, leaving many people wary, said Aykan Erdemir, former member of the Turkish Parliament.
"Turkey is known to host these big parties to celebrate New Year's, but this year most citizens were wary of Islamist attacks so they chose to celebrate it at home with their friends," he said.
"This is an attack on the Western lifestyle. This is an attack on Turkey's secular, urban way of living. And this will simply fuel the ongoing cultural clashes, the ongoing polarization in Turkey," Erdemir said.
Mehmet Gormez, president of Religious Affairs, Turkey's religious authority, said that the attack was "savagery, horrendous" and that the "only nuance which differentiates the terrorist action committed tonight from others is that it aims to divide the nation and confront people from different lifestyles by inciting them."
The US State Department said attacks like this one on its NATO ally "only reinforce our strong determination to work with the government of Turkey to counter the scourge of terrorism."
A violent year
ISIS and Kurdish militants both have staged attacks in Turkey, which is still reeling from a bloody and failed military coup in July.
ISIS is suspected in a June attack at Istanbul's Ataturk Airport that left 44 people dead and an explosion at an August wedding, not far from the border with Syria, that killed at least 54 people.
Meanwhile, Turkish security forces clash almost daily with Kurdistan Workers Party, or PKK, militants, mostly in predominantly Kurdish parts of southeastern Turkey.
A pair of bombings in Istanbul killed 44 people and wounded 155 others December 10 in an attack by a breakaway group of the PKK. The two explosions occurred after a heavily attended soccer game at Besiktas Vodafone Arena.
On December 17, a car bomb exploded near a public bus, killing 13 soldiers in the central province of Kayseri. Three days later, a gunman assassinated Russia's ambassador to Turkey at an Ankara art gallery.