New Year's Celebrations in a Turkish nightclub ended in a bloodbath when a gunman dressed as Santa Claus opened fire killing at least 39 people and injuring dozens more.
Turkish interior minister Suleyman Soylu said 15 foreign nationals had been identified among the dead at the Reina nightclub in Istanbul.
There had been up to 600 people in the club at the time of the attack.
Authorities are starting to uncover evidence about the attack but there is no clarity yet on who was responsible, Turkish Prime Minister Binali Yildirim said.
"Some details have started emerging, but the authorities are working towards a concrete result," Yildirim told reporters, when asked about who might have been behind the attack.
The "terrorist" was still at large after striking alone in the attack, in which he changed clothes and injured a further 69 people in the early hours of Sunday, Mr Soylu said.
"Police and security officials will share information as it becomes available during the investigation," he said.
"A manhunt for the terrorist is underway. Police have launched operations. We hope the attacker will be captured soon," he told reporters.
"This was a massacre, a truly inhuman savagery," he added.
Foreign Affairs Minister Charlie Flanagan has said there are no reports of Irish casualties but has urged holiday-makers to remain vigilant.
"I unreservedly condemn the heinous violent attack in the nightclub in Istanbul in the early hours of this morning. I extend my condolences to the families of those killed and my sympathy to those injured.
"My thoughts are with the people of Turkey today, as they come to terms with yet another terrorist attack on their country. It is particularly heartrending that this attack targeted people on New Year’s Eve, a time when we join together in our hope for a more peaceful future," he said.
"Terrorists will not prevail in destabilising democracy as long as we continue to stand together and I reiterate that Ireland, along with other European Union Member States, stands in solidarity with Turkey in the fight against terrorism.
"Ireland's Embassy in Ankara is in ongoing close contact with local Turkish authorities and other partners. We have not received any report of any Irish casualties.
“I advise any Irish citizens in Istanbul to remain very vigilant and to exercise extreme caution, to follow the advice of local authorities especially the police, and to avoid crowded areas and those frequented by foreigners."
Nationals of Saudi Arabia, Morocco, Lebanon and Libya were among those killed in a gun attack at a packed nightclub in Istanbul on Sunday, Turkish Family Minister Fatma Betul Sayan Kaya was quoted by the state-run Anadolu news agency as saying.
A woman from Israel, aged 19, was among those killed it has emerged.
The Israeli Foreign Ministry confirmed on Sunday that the teenager was among the casualties.
She was identified as Leanne Nasser, 19. Israeli media reports said she had been at the club with three friends from her home village of Tira in central Israel, one of whom was wounded in the shooting attack.
At least five Saudi citizens were among 39 people killed in an attack on New Year revellers at a packed nightclub in Istanbul, a Saudi newspaper reported.
Meanwhile, two Lebanese citizens were also among those killed.
In a statement at the weekly cabinet meeting on Sunday, Israeli Prime Minister Benjamin Netanyahu said: "We saw over the weekend ... another deadly attack in Turkey. We send condolences to the families of those who were murdered and best wishes for recovery to the wounded."
Meanwhile, Turkish President Erdogan says Turkey is determined to "destroy source of threats and attacks against it".
"As a nation, we will fight to the end against not just the armed attacks of terror groups and the forces behind them, but also against their economic, political and social attacks," Erdogan said in a written statement.
"They are trying to create chaos, demoralize our people, and destabilize our country with abominable attacks which target civilians ... We will retain our cool-headedness as a nation, standing more closely together, and we will never give ground to such dirty games," he said.
Istanbul governor Vasip Sahin said the "terror attack" began when the assailant, armed with a long-barrelled weapon, killed a policeman and a civilian outside the club.
The attacker then entered the club in the Ortakoy district and fired at partygoers at around 1.45am local time, Mr 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," he added.
Five of the dead have been identified as Turkish nationals, up to four of which were working in the club, while authorities are yet to identify 19 others.
Clubbers reportedly jumped into the waters of the Bosporus strait to escape to flee the attacker.
Eyewitness Sinem Uyanik said her husband Lutfu Uyanik was wounded in the attack.
"Before I could understand what was happening, my husband fell on top me," she said outside Istanbul's Sisli Hospital.
"I had to lift several bodies from on top of me before I could get out."
A Turkish soccer player named Sefa Boydas wrote on Twitter "I didn't see who was shooting but heard the gun shots and people fled. Police moved in quickly."
"My girlfriend was wearing high heels. I lifted her and carried her out on my back," he said.
Hurriyet quoted Reina's owner, Mehmet Kocarslan, as saying security measures had been taken over the past 10 days after U.S. intelligence reports suggested a possible attack.
Officers in riot gear and armed with machine guns were backed up by armoured vehicles to secure the area surrounding the club, which is one of the most popular spots in the city.
Security in Turkish cities had been heightened with 17,000 police officers, some camouflaged as Santa Claus, on duty in Istanbul, state news agency Anadolu said.
Terrorists from the Islamic State group and Kurdish rebels targeted Istanbul and Ankara several times inflicting a death toll of more than 180 in 2016.
Turkish-backed rebel groups in Syria have been targeting IS militants.
The White House condemned the "horrific terrorist attack" and offered US help to Turkey.
The Irish embassy in Ankara appealed for Irish holiday-makers to let their loved ones know they are safe.
If you are in Istanbul and ok, pls let family and friends know. For assistance call 00353 1 408 2000 or 0090 312 459 1000.— Irish Embassy Ankara (@IrlEmbAnkara) January 1, 2017
Minister for Foreign Affairs Charlie Flanagan has extended his condolences to the family and friends of those killed and injured in the attack.
Shocking start to New Year 2017 with terror attack in Istanbul Our thoughts are with victims, survivors, their families & communities.— Charlie Flanagan (@CharlieFlanagan) January 1, 2017
Any Irish citizen in Istanbul that needs assistance, or anyone in Ireland concerned about someone in Istanbul, should call on 00353 1 4082000 or 0090 312 459 1000.