Authorities searched Sunday for a gunman who opened fire at a packed nightclub during New Year's celebrations in Istanbul, killing at least 39 people and injuring 69.

In what Istanbul Gov. Vasip Sahin described as a terror attack, an assailant reportedly dressed as Santa Claus shot and killed a policeman and civilian outside the Reina club around 1:15 a.m. local time before entering and shooting up the club.

More than 500 people were inside the club at the time, private NTV television reported. Some patrons jumped into the Bosphorus Strait to escape.

"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.

Nobody immediately claimed credit for the attack, which Turkish authorities said was carried out by a lone assailant. But President Recep Tayyip Erdoğan said the country is "extremely determined to do whatever it takes" after recent terror attacks to secure the region.

"Those aiming at the peace of our nation, and their pawns, are working to destabilize our country and trigger chaos by demoralizing our people through their heinous attacks that also target civilians," Erdogan said. "We are aware that these attacks, carried out by various terror organizations against our country, are not independent from incidents happening in our region. We are determined to eradicate threats and attacks against our country at their source."

Reina owner Mehmet Kocarslan, told the private Dogan news agency, that police had boosted security measures in the upscale neighborhood of Ortakoy and its vicinity. The efforts included a 24-hour police presence and complementary efforts by the coast guard at sea.

“Despite all these precautions by police forces, unfortunately this painful event took place,” he said. “We are at the point where all words end.”

State-run Anadolu news agency reported Sunday that a 22-year-old police officer, Burak Yildiz, was shot and killed outside the Reina nightclub.

Sahin said the attacker used a long-barreled weapon, but he gave no other details about the device.

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.

Turkey’s Minister of Family and Social Policies, Fatma Betul Sayan Kaya, told reporters outside a hospital that foreigners are among the wounded.

“There are many different nationalities, Saudi Arabia, Morocco, Lebanon, Libya and citizens of other nations,” Kaya said.

Erdogan condemned the attack and expressed his condolences to victims of the attack in a series of tweets.

The White House issued a statement condemning the attack.

"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.

Pope Francis decried attack on what he called a “night of good-wishes and hope.” Francis told pilgrims and tourists in St. Peter’s Square Sunday that he was close in prayer to the many dead and to their families, to the wounded and to the entire Turkish people.

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.”