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USA TODAY

An assailant believed to be dressed as Santa Claus opened fire in an Istanbul nightclub jammed with New Year's Eve revelers early Sunday, killing at least 39 and wounding at least 69 others, authorities said.

The city's governor described the mass shooting as a terror attack, the latest to shake Turkey in a year of bloodshed, and it was not immediately clear whether one attacker or several participated. Gov. Vasip Sahin said the attacker, armed with a long-barreled weapon, killed a policeman and a civilian outside the club before entering and firing on people partying inside.

"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," Sahin told reporters.

Authorities launched a search for the attacker, who has not been identified.

“Our security forces have started the necessary operations. God willing he will be caught in a short period of time,” said Interior Minister Suleyman Soylu.

At least 15 of the dead were foreign nationals, Soylu said. He did not identify their nationalities. Five of the victims were identified as Turkish nationals. Authorities were trying to identify the rest.

The White House issued a statement condemning the attack.

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"That such an atrocity could be perpetrated upon innocent revelers, many of whom were celebrating New Year's Eve, underscores the savagery of the attackers. We offer our thoughts and prayers to the families and loved ones of those killed, and a speedy recovery to the wounded," the statement read.

Turkish security forces have been on particularly high alert in the past weeks. On Dec. 20, the Russian ambassador to Turkey was killed while attending an art exhibition.

The Russian diplomat, Andrei Karlov, was shot dead by off-duty Turkish policeman Mevlut Mert Altintas as he was giving a speech in Ankara, the capital. The killer shouted that the murder was in retaliation for Russian involvement in the ongoing conflict in the Syrian city of Aleppo.

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In the latest attack, media reports say the assailant entered the popular Reina nightclub, in Istanbul's Ortakoy district on European side of the Bosphorus River, at 1:45 a.m. local time, eight hours ahead of Eastern Time, dressed in a Santa Claus costume. More than 500 people were inside the club at the time, private NTV television reported.

A New York Times report suggested the possibility of multiple attackers, and no one has been arrested.

Among many tweets posted about the shooting was one from Israel News Online, which included a short, grainy video clip that appeared to show a man wearing a Santa hat clutching what appears to be a weapon.

Some customers jumped into the waters of the Bosporus to escape.

Footage from the scene outside the Reina nightclub showed at least six ambulances with flashing lights and civilians being escorted out. NTV said police had cordoned off the area and an operation to capture the assailant was ongoing.

“Before I could understand what was happening, my husband fell on top me,” Sinem Uyanik said outside Istanbul’s Sisli Etfal Hospital where she waited to see her husband wounded in the attack. “I had to lift several bodies from top of me before I could get out. It was frightening.”

Her husband was not in serious condition despite sustaining three wounds.

An AP photographer said police cordoned off the area about 2 miles away from the nightclub and reported multiple ambulances passing by.

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Police with riot gear and machine guns backed up with armored vehicles blocked the area close to the nightclub, one of the most popular night spots in Istanbul.

This attack is, of course, a horrible development, but not shocking to many Turks who chose to stay inside this New Year's Eve," Aykan Erdemir, former member of the Turkish Parliament, told CNN. "Turkey is known to host these big parties to celebrate New Year's. But this year most citizens were wary of Islamist attacks, so they chose to celebrate it at home with their friends."

He said the attack seemed similar to what happened at the Bataclan, a Paris concert hall that gunmen attacked in 2015.

"This is an attack on the Western lifestyle. This is an attack on Turkey's secular, urban way of living. And this will simply fuel the ongoing cultural clashes, the ongoing polarization in Turkey," Erdemir said.

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.

Ankara and Istanbul have been targeted by several attacks in 2016 carried out by the Islamic State group or Kurdish rebels, killing more than 180 people.

Contributing: AP reporters Zeynep Bilginsoy in Istanbul and Suzan Fraser in Ankara