ISTANBUL — An assailant believed to have been dressed in a Santa Claus costume opened fire at a nightclub in Istanbul during New Year's celebrations, killing at least 35 people and wounding 40 others in what the province'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 at around 1:45 a.m. Sunday before entering and firing on people partying inside. He did not say who may have carried out the shooting but called it a “terrorist attack.”

There was no immediate claim of responsibility.

“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.

Sahin mentioned only one assailant, but witnesses, speaking to local media, said they had seen two or more.

Private NTV news channel said at least one assailant entered the Reina nightclub, in Istanbul's Ortakoy district, dressed in a Santa Claus outfit.

Some customers jumped into the waters of the Bosporus to escape the attack, the report said.

The whereabouts of the assailant or assailants were not known Sunday and it was not clear if anyone had been caught.

Witnesses described the crowd at the waterside club being sprayed with gunfire, and stampedes of panicked patrons scrambling for cover.

Sinem Uyanik was inside the club with her husband who was wounded in the attack.

“Before I could understand what was happening, my husband fell on top of me,” she said outside Istanbul's Sisli Etfal Hospital. “I had to lift several bodies from on top of me before I could get out. It was frightening.”

Her husband was not in serious condition despite sustaining wounds.

Amid heavy rain, police in riot gear, with assault rifles and backed up by armored vehicles blocked the area close to the nightclub, one of the most popular night spots in Istanbul. Several ambulances flashing blue lights arrived on the scene, some taking wounded to hospitals.

The White House condemned what it called a “horrific terrorist attack” and offered U.S. help to Turkey.

White House spokesman Eric Schultz said President Barack Obama was briefed on the attack by his national security team and asked to be updated as the situation developed. Obama is vacationing in Hawaii with his family.

White House National Security Council spokesman Ned Price said the attack on “innocent revelers” celebrating New Year's shows the attackers' savagery. He said the U.S. was sending thoughts and prayers to the relatives of those killed.

Price also said the U.S. supports its NATO ally Turkey as both fight terrorism.

At the time of the attack, an estimated 600 people were celebrating inside the club that is also frequented by famous locals, including singers, actors and sports stars. Several shocked revelers were seen fleeing the scene after the attack and the music fell silent.

The club is located close to the site of recent suicide attacks that killed dozens near a soccer stadium.

Its owner, Mehmet Kocarslan, told the Turkish newspaper Hurriyet that there had been increased security at the club for a week or more, because of warnings from “American intelligence.” He told the paper there was more than one gunman and that they used Kalashnikov rifles.

Ahead of the attack, security measures had been heightened in major Turkish cities, with police barring traffic leading to key squares in Istanbul and the capital, Ankara. In Istanbul, 17,000 police officers were on duty, some camouflaged as Santa Claus and others as street vendors, state news agency Anadolu reported.

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

Justice Minister Bekir Bozdag vowed that Turkey would press ahead with its fight against violent groups.

“Turkey will continue its determined and effective combat to root out terror,” Bozdag said on Twitter.

The Washington Post contributed to this report.