Police in Istanbul are hunting for a gunman who opened fire at a nightclub, killing at least 39 people.
The attack happened at Reina nightclub early on Sunday, as hundreds of revellers marked the new year.
Officials say at least 15 foreigners were killed, including an Israeli woman. Citizens from Morocco, Lebanon, Saudi Arabia and Libya were hurt.
The attacker left his gun at the scene before fleeing, the Turkish Prime Minister Binali Yildirim said.
He also confirmed the gunman was not dressed as Santa Claus, contradicting earlier reports.
At least 69 people are being treated in hospital, officials said, with four in a serious condition.
The motive for the attack is not clear. President Recep Tayyip Erdogan blamed terror groups “trying to create chaos”.
“They are trying to… demoralise our people and destabilise our country,” he said.
In an early morning statement, Interior Minister Suleyman Soylu described the attack as “a massacre, a truly inhuman savagery”.
“A manhunt for the terrorist is under way. Police have launched operations. We hope the attacker will be captured soon.”
Police believed there was only one gunman, he said, although some eyewitness reports mentioned multiple attackers.
IS threats: By Rengin Arslan, BBC Turkish, Istanbul
Despite there being no official statement about who might be behind this brutal attack, the finger of blame is being pointed at the so-called Islamic State.
In the last two years of attacks in Turkey, Kurdish militants have mostly targeted military forces and police, while IS is known to target civilians.
IS leaders have threatened Turkey and called on their followers to carry out attacks inside the country.
Turkey began a ground operation against IS as well as Kurdish groups inside Syria four months ago.
Early on Sunday, the interior minister said some of the dead had yet to be identified. Israel has confirmed 19-year-old Leanne Nasser was killed.
Turkish state news agency Anadolu also quoted Family Minister Fatma Betul Sayan Kaya as saying most of the victims were foreigners “from different countries – Saudi Arabia, Morocco, Lebanon, Libya”. Later reports clarified that she was referring to the injured, rather than to the dead.
A policeman and a security guard were shot outside the club as the attack began. The security guard has been identified as Fatih Cakmak. His brother said he was on duty when Kurdish militants attacked a football stadium in December, killing 38 people, mostly police.
Reina nightclub, in the the Ortakoy area of Istanbul, is an upmarket venue on the banks of the Bosphorus.
There were reportedly as many as 700 people in the nightclub at the time of the attack, some of whom jumped into the water to escape.
Eyewitnesses described panic and confusion as the attack unfolded.
“Gunshots rang out. When those sounds were heard, many girls fainted,” professional footballer Sefa Boydas told AFP news agency.
He said people appeared to be crushed as they ran away. “They say 35 to 40 died but it’s probably more because when I was walking, people were walking on top of people.”
US President Barack Obama, who is on holiday in Hawaii, offered condolences “for the innocent lives lost” and offered “appropriate assistance”.
Russian President Vladimir Putin condemned the “cynical” murder of civilians. “Our shared duty is to decisively rebuff terrorist aggression,” he said.
Turkey and Russia are working together on efforts to end the fighting in Syria, though they support different sides in the conflict.
Istanbul was already on high alert with some 17,000 police officers on duty in the city, following a string of terror attacks in recent months.
Many were carried out by so-called Islamic State (IS) or Kurdish militants.
Deadly attacks in Turkey in 2016
10 December: Twin bomb attack outside a football stadium in Istanbul kills 44 people, Kurdish militant group claims responsibility
20 August: Bomb attack on wedding party in Gaziantep kills at least 30 people, IS suspected
30 July: 35 Kurdish fighters try to storm a military base and are killed by the Turkish army
28 June: A gun and bomb attack on Ataturk airport in Istanbul kills 41 people, in an attack blamed on IS militants
13 March: 37 people are killed by Kurdish militants in a suicide car bombing in Ankara
17 February: 28 people die in an attack on a military convoy in Ankara
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