A huge manhunt is taking place in Turkey after a gunman targeted a crowded Istanbul nightclub during New Year celebrations, killing at least 39 people before fleeing.
Foreigners were among those killed, including an 18-year-old Israeli woman, three Indian citizens, a 26-year-old man from Lebanon and a Belgian national.
Close to 70 others were injured in what authorities described as a terror attack. Three of the wounded were in a critical condition.
President Recep Tayyip Erdogan vehemently condemned "the terror attack in Istanbul's Ortakoy neighbourhood in the first hours of 2017" and offered condolences for those who lost their lives, including "foreign guests".
The attacker, armed with a long-barrelled weapon, killed a policeman and a civilian outside the popular Reina club at around 1.15am before entering and firing on people partying inside, Governor Vasip Sahin said.
"Unfortunately, (he) rained bullets in a very cruel and merciless way on innocent people who were there to celebrate New Year's and have fun," Mr Sahin told reporters.
There was no immediate claim of responsibility for the attack and authorities did not name any suspects. The bloodiest attacks that Turkey endured in 2016 were the work of the Islamic State group or Kurdish militants.
Turkey is a member of Nato and a partner in the US-led coalition against IS. The country is also facing renewed conflict with Kurdish rebels in the south east, and across the border in Syria and Iraq.
Interior Minister Suleyman Soylu said a suspect has not been identified and that the gunman remains at large. Mr Soylu, describing the attack as a "massacre, a truly inhumane savagery", said three or four of the Turkish victims may have been employees at the nightclub.
"Our security forces have started the necessary operations. God willing, he will be caught in a short period of time," Mr Soylu said.
Private NTV news channel said the assailant entered the upmarket nightclub, on the shores of the Bosporus, on the European side of the city, dressed in a Santa Claus outfit.
Security camera footage obtained by The Associated Press from Haberturk newspaper shows the gunman dressed in black and carrying a backpack as he shoots down a police officer outside the Reina nightclub.
Footage taken by a different camera shows him inside the venue wearing different clothes and a Santa Claus hat.
However, Turkey's prime minister denied that the gunman wore a Santa Claus outfit.
"There is no truth to this. He is an armed terrorist as we know it," Prime Minister Binali Yildirim said
He said the attacker left a gun inside the venue and escaped by "taking advantage of the chaos" that ensued.
Some customers reportedly jumped into the waters of the Bosporus to escape the attack.
Mehmet Dag, 22, was passing by the club and saw the suspect shoot at a police officer and a bystander. He said the suspect then targeted security guards, gunning them down and entering the club.
"Once he went in, we don't know what happened. There were gun sounds, and after two minutes the sound of an explosion," Mr Dag said.
Turkish media said the victims include a 22-year-old police officer and a 47-year-old travel agent, both of whom were shot outside the club.
The nightclub area remained sealed off on Sunday afternoon.
Heavily armed police blocked the snowy street in front of the nightclub where the entrance was covered with blue plastic sheeting below a Turkish flag. Police patrolled the Asian side of the Bosporus on the other side of the club.
Turkey's Minister of Family and Social Policies Fatma Betul Sayan Kaya said citizens of Saudi Arabia, Morocco, Lebanon and Libya were among those hurt in the attack.
The US Consulate General in Istanbul on Sunday warned American citizens to keep their movements in the city "to an absolute minimum".
A statement reminded US citizens that extremists "are continuing aggressive efforts to conduct attacks in areas where US citizens and expatriates reside or frequent".
The United States also denied reports in Turkish new outlets and on social media that its security agencies knew in advance that the nightclub in Istanbul was at risk of a terror attack.
The US Embassy in Ankara said that "contrary to rumours circulating in social media, the US Government had no information about threats to specific entertainment venues, including the Reina Club."
Major attacks carried out by IS or Kurdish militants killed more than 180 people in Istanbul and Ankara alone in 2016.
On December 10, a double bomb attack outside a soccer stadium near the Reina nightclub killed 45 people and wounded some 150 others. The attack was claimed by Turkey-based Kurdish militant group, the Kurdish Freedom Falcons.
"Turkey continues its combat against terror and is absolutely determined to do whatever is necessary in the region to ensure its citizens' safety and peace," Mr Erdogan said.
The UN Security Council condemned the attack "in the strongest terms", calling the assault "a heinous and barbaric terrorist attack".
In a statement, the council members also expressed sympathy and condolences to the families of those killed and of the dozens of those wounded.
The council said it reaffirms "that terrorism in all its forms and manifestations constitutes one of the most serious threats to international peace and security".