Turkey was once again the focal point of a deadly terrorist attack, as at least one assailant believed to be dressed in a Santa Claus costume opened fire at a nightclub in Istanbul during New Year's celebrations early Sunday.

At least 35 people are dead and 40 others were wounded in what the city's governor described as a terror attack.

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. He did not say who may have carried out what he called a "terror attack."

"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.

Turkish security forces have been on particularly high alert over 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 the capital Ankara. The killer shouted that the murder was in retaliation for Russian involvement in the ongoing conflict in the Syrian city of Aleppo.

Media reports said 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 that there could have been multiple attackers.

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.

An AP photographer said police cordoned off the area about 2 miles (3 kilometers) away from the nightclub and reported multiple ambulances passing by.

"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 was attacked by gunmen 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: Suzan Fraser in Ankara, and USA TODAY reporter Marco della Cava