ISTANBUL (AP) -- The Islamic State group on Monday made an unusual claim of responsibility for a major terrorist attack in Turkey, saying a "soldier of the caliphate" carried out the mass shooting at an Istanbul nightclub that killed 39 people as they welcomed the new year.
The group said Christian revelers were targeted in response to Turkish military operations against IS in northern Syria, but most of the dead were foreign tourists from Muslim countries.
The claim came after a recent IS propaganda video urged attacks on Turkey, which is home to an air base used in the U.S.-led effort against the group in Syria and Iraq.
Turkish authorities never confirmed the authenticity of the Dec. 22 video that purported to show Turkish soldiers who were burned alive, but access to social media was temporarily restricted in what appeared to be an effort to curb circulation of the footage.
The nightclub assailant, armed with a long-barreled weapon, killed a policeman and a civilian early Sunday outside the Reina club before entering and firing at some of the estimated 600 people inside. The establishment is frequented by famous locals, including singers, actors and athletes.
Authorities obtained the fingerprints and a basic description of the gunman and are close to identifying him, Deputy Prime Minister Numan Kurtulmus said Monday after a weekly cabinet meeting. He confirmed that eight people have been detained in connection with the attack.
Other attacks in Turkey have been linked to IS, but without specific claims of responsibility.
For some analysts, the claim signaled a shift in IS strategy in Turkey, a predominantly Muslim nation and NATO member.
"It's a new phase," security analyst Michael Horowitz said. "What we saw before was an undeclared war, and now we're entering an open war."
The IS claim said only that the attacker struck to "let infidel Turkey know that the blood of Muslims that is being shed by its airstrikes and artillery shelling will turn into fire on its territories."
Early Turkish media reports suggested the nightclub gunman was probably from either Uzbekistan or Kyrgyzstan and may have been part of the same cell that staged a June attack on Istanbul Ataturk Airport that killed 45 people.
By attacking as the nation was celebrating the new year, the group indicated that it intends to continue being a "scourge" against Turkey in 2017, Kurtulmus said.
In its claim, IS said the nightclub attack was aimed at Christians celebrating a pagan holiday, suggesting a symbolic choice of target that can be justified to radical Sunni Muslim supporters as punishment of sinners. But in reality, many of the victims hailed from majority-Muslim nations in the Middle East.
Max Abrahms, a Northeastern University political scientist, said IS understands that civilian attacks can be counterproductive in countries where it has abundant support. To him, the change of tact in Turkey reflects the mindset of IS in the wake of losses in Syria and Iraq.
"There's no question that Islamic State is suffering in an irreversible way," Abrahms said. So the group wants to commit as many attacks as possible and "is much more likely to claim credit for them in order to signal that it has continued capability to mount operations around the world."