A manhunt for the gunman who killed 39 people in an Istanbul night club early Sunday continued into the evening, but authorities still have "no clarity" on who was responsible, Turkish Prime Minister Binali Yildrim said.

"Some details have started emerging, but the authorities are working towards a concrete result," Yildrim told reporters Sunday evening.

There was no immediate claim of responsibility for the attack on the nightclub that was packed with New Year's revelers.

Some reports said the gunman was dressed as Santa Claus, but Prime Minister Yildrim denied them.

Interior Minister Suleyman Soylu and Vasip Sahin, Istanbul's governor, said the assault on the club was carried out by a lone gunman.

Among those identified, 15 of the dead are foreign nationals, five are Turkish, Soylu said, adding that a police officer was also killed. Eighteen victims are yet to be identified.

An ambulance arrives near a nightclub where a gun An ambulance arrives near a nightclub where a gun attack took place during a New Year's party in Istanbul, Turkey, Jan. 1, 2017. An ambulance arrives near a nightclub where a gun attack took place during a New Year's party in Istanbul, Turkey, Jan. 1, 2017.

Soylu said 69 people are being treated in hospitals for injuries, four of them are serious and one very serious.

City governor Sahin called the incident a terror attack. He said the attacker used a long-barreled weapon to brutally and savagely shoot revelers.

Turkish president Recep Tayyip Erdogan said the attacker wanted to create pandemonium in the country but failed.

"They are trying to create chaos, demoralize our people, and destabilize our country with abominable attacks which target civilians," President Erdogan said. "We will retain our cool-headedness as a nation, standing more closely together, and we will never give ground to such dirty games."

Turkish police converged on the area around the Reina club in Istanbul’s upscale Ortakoy area, an entertainment spot on the Bosporus that is popular with celebrities and foreigners.

Witnesses to attack

Witnesses said as many as 600 people were inside at the time of the attack and said some revelers jumped into the waterway to escape the gunfire. Local reports say the police officer and a civilian were gunned down outside the facility before the gunman gained entry.

Aziz Ozcan, whose brother Suleyman was working at the bar section of the nightclub, told VOA's Turkish service he was working elsewhere when he heard of the attack.

"My father and my mother are old, they can’t come here so I came down here. But I don’t know what to do. We didn’t receive any information. ... I don’t know if he is alive or not. ... We are just waiting," Ozcan told VOA's Turkish service.

“We were having fun. All of a sudden people started to run. My husband said don’t be afraid, and he jumped on me. People ran over me. My husband was hit in three places,” club-goer Sinem Uyanik told The Hurriyet newspaper.

“I managed to push through and get out, it was terrible,” Uyanik said, describing seeing people soaked in blood.

“I didn’t see who was shooting but heard the gun shots and people fled. Police moved in quickly,” Sefa Boydas, a Turkish soccer player, wrote on Twitter.

“My girlfriend was wearing high heels. I lifted her and carried her out on my back,” Boydas said.

The Turkish government imposed a temporary blackout on coverage of the attack, citing national security reasons.

Video posted on social media showed rescue vehicles and police cars rushing to the scene right after the attack was first reported. Wounded people were seen being taken into ambulances.

People run away from a nightclub where a gun attac People run away from a nightclub where a gun attack took place during a New Year's Eve party in Istanbul, Turkey, Jan. 1, 2017. People run away from a nightclub where a gun attack took place during a New Year's Eve party in Istanbul, Turkey, Jan. 1, 2017.

The 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.

Obama briefed

Principal Deputy White House Press Secretary Eric Schultz said U.S. President Barack Obama has been briefed on the Istanbul attack by his national security team.

“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,” Schultz said.

Obama also reaffirmed U.S. support for NATO ally Turkey.

In a statement from the U.S. State Department, Deputy spokesman Mark Toner said: “These attacks only reinforce our determination to work with the government of Turkey to counter the scourge of terrorism.”

Watch: Istanbul Terror Attack: Interviews With Eyewitness, Family Members

Security measures have been upgraded in major Turkish cities following a spate of terror attacks in recent months.

Two explosions near an Istanbul football stadium earlier this month killed 38 people and wounded more than 150 others.

A Kurdish militant group later claimed responsibility for that attack.

In late June, a separate attack claimed by Islamic State extremists killed more than 40 people at Istanbul’s Ataturk airport.