Turkish police have released images of the prime suspect in the Istanbul nightclub terror attack.
A manhunt is underway for the gunman after he opened fire on New Year revellers, killing at least 39 people, including many foreigners.
Almost 70 people were also wounded in the attack on the popular night spot, Turkey's interior minister Suleyman Soylu has said.
He said they were all in hospital and four of them were in a critical condition.
"A manhunt for the terrorist is underway. Police have launched operations. We hope the attacker will be captured soon," he told reporters.
Authorities have said that the gunman left his weapon at the scene and they are hoping it could reveal more clues about the attacker's identity.
The man has been described as possibly being from Afghanistan, Chechnya or East Turkestan - a region of western China.
Funeral services have already been held for some of the victims and tributes have been laid in front of the club.
Turkey's state-run news agency Anadolu Agency has reported that 24 of the shooting attack's victims were from other countries, including Saudia Arabian, Iraqi, Isreali and Belgian nationals.
Anadolu says authorities still are trying to identify four of the victims.
A spokeswoman for the Australian Department of Foreign Affairs and Trade said the department "has not received any advice of Australians affected by the attack in Istanbul".
The attacker shot a police officer and a civilian as he entered Reina bar in the Turkish metropolis before opening fire inside the building, Governor Vasip Sahin said.
It's being reported the gunman was dressed as Santa Claus.
Some people jumped into the adjacent Bosphorus waterway to save themselves after the attacker began shooting at random in the Reina nightclub just over an hour into the new year on Sunday.
Witnesses described diving under tables as the assailant walked around spraying bullets from an automatic rifle.
The nightclub was located in a wealthy part of Istanbul and frequented by many foreign visitors, celebrities and local elites.
President Recep Tayyip Erdogan said the attack was orchestrated to create chaos in Turkey.
"They are working to destroy our country's morale and create chaos by deliberately targeting our nation's peace and targeting civilians with these heinous attacks," Erdogan said in a statement on the presidency website, adding he would redouble efforts to combat terrorism in Turkey.
"Turkey is determined to continue to fight to the end against terror and to do whatever is necessary to ensure the security of its citizens and secure peace in the region," he said.
A police officer is among the dead, Governor Sahin stated.
"A terrorist with a long-range weapon... brutally and savagely carried out this incident by firing bullets on innocent people who were there solely to celebrate the New Year and have fun," Mr Sahin told reporters at the scene.
"At first we thought some men were fighting with each other," said a Lebanese woman called Hadeel who was in the club with her husband and a friend. "Then we heard the sound of the gunfire and ducked under the tables.
"We heard the guy screaming Allahu Akbar (God is Greatest), all three of us heard that ... We heard his footsteps crushing the broken glass," she told Reuters.
"We got out through the kitchen, there was blood everywhere and bodies."
Initial reports suggested that two gunmen had stormed the building, but authorities are currently understood to be searching for one killer.
The Turkish authorities have imposed a media blackout on coverage of the attack.
There were reportedly as many as 500 people in the nightclub at the time.
Istanbul had been on high alert with some thousands of police officers on duty in the city following a recent string of terror attacks across Turkey.
Istanbul has been the target in a series of attacks by Islamic State and Kurdish extremist groups including one bloody assault at the city's main airport in June that left 41 people dead.
Security measures had been heightened in major Turkish cities, with police barring traffic leading up to key squares in Istanbul and the capital Ankara.
In Istanbul, 17,000 police officers were put on duty, some camouflaged as Santa Claus and others as street vendors, state news agency Anadolu reported.
Turkish police officers dressed as Santa Claus to provide security in İstanbul tonight. 🎅🏼🎅🏼🎅🏼🎅🏼🎅🏼 pic.twitter.com/gxObhUXW1J
— Ragıp Soylu (@ragipsoylu) December 31, 2016
The nightclub owner Mehmet Kocarslan told Turkey's Hurriyet news site that security at the venue had been increased over the past 10 days following warnings from American intelligence officers, according to the Daily Mail.
He stated that the attack had been carried out with a Kalashnikov rifle.
US President Barack Obama was among the first international leaders to make a statement.
"The president expressed condolences for the innocent lives lost, directed his team to offer appropriate assistance to the Turkish authorities, as necessary, and keep him updated as warranted," White House spokesman Eric Schultz said in a statement.