ISTANBUL - An assailant believed to have been dressed in a Santa Claus costume opened fire at a crowded nightclub in Istanbul during New Year's celebrations, killing at least 39 people and wounding close to 70 others in what the province's governor described as a terror attack.

Gov. Vasip Sahin said the attacker, armed with a long-barrelled weapon, killed a policeman and a civilian outside the club at around 1:15 a.m. Sunday before entering and firing on people partying inside.

He did not say who may have carried out the 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. Private NTV news channel said the assailant entered the Reina nightclub, in Istanbul's Ortakoy district, dressed in a Santa Claus outfit. Interior Minister Suleyman Soylu said the man was still on the run, adding: “efforts to find the terrorist are continuing.”

“Our security forces have started the necessary operations. God willing he will be caught in a short period of time,” the minister said. At least 15 of the dead were foreign nationals, Soylu said, without providing information on their nationalities. Five of the victims were identified as Turkish nationals while authorities were still trying to identify the rest of the victims. At least 69 people were being treated in hospitals, four in serious condition, Soylu said.

Three or four of the Turkish victims may have been employees at the nightclub, he said. “This was a massacre, a truly inhuman savagery,” Soylu said. The minister said the attacker was believed to have left the club wearing “different clothing” to those he entered the club in. He said the attacker is believed to have carried out the assault alone. “Four of the injured are in very serious condition,” said Health Minister Recep Akdag.

“There are several foreign nationals among the wounded.” Some customers jumped into the waters of the Bosporus to escape the attack, the report said. 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 me,” she saidoutside Istanbul's Sisli Etfal Hospital. “I had to lift several bodiesfrom top of me before I could get out. It was frightening.”

Her husband was not in serious condition despite sustaining three wounds. Police with riot gear and machine-guns backed up by armoured vehicles blocked the area close to the Reina 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 this week with his family. National Security Council spokesman Ned Price said the attack on “innocent revellers” celebrating New Year's shows the attackers' savagery.

“Our thoughts are with victims and their loved ones. We continue to work to prevent these tragedies,” European Union foreign policy chief Federica Mogherini tweeted. NATO Secretary General Jens Stoltenberg said on Twitter: “Tragic start to 2017 in Istanbul. My thoughts are with those affected by the attack on people celebrating New Year and with the Turkish people.” An estimated 600 people were celebrating inside the club that is also frequented by famous locals, including singers, actors and sports stars.

Several shocked revellers were seen fleeing the scene after the attack and the music fell silent. The country has been rocked by a series of deadly attacks in 2016 carried out by the Islamic State group or Kurdish militants, killing more than 180 people. On Dec. 10, a double bomb attack outside soccer stadium - located near the Reina nightclub - killed 44 people and wounded 149 others.

The attack was claimed by Turkey-based Kurdish militant group, the Kurdish Freedom Falcons. Nine days later, an off-duty Turkish riot policeman assassinated Russian Ambassador Andrei Karlov at a photo exhibition in the capital, Ankara. The government has suggested that a movement led U.S.-based Muslim cleric Fethullah Gulen was behind the killing - an accusation the cleric has denied. 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.

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, Turkey's Anadolu news agency reported.

Associated Press writers Dusan Stojanovic and Mehmet Guzel in Istanbul and Suzan Fraser in Ankara, Turkey contributed to this report.