At least one U.S. citizen was injured during a deadly attack on an Istanbul nightclub -- and the terrorist responsible for the massacre remained at large Sunday evening.

At least 39 people were killed and nearly 70 were wounded when a gunman opened fire shortly after a nightclub crowd had celebrated ringing in the New Year.

"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," Gov. Vasip Sahin told reporters.



The attacker, armed with a long-barreled weapon, killed a policeman and a civilian outside the popular Reina club at around 1:15 a.m. before entering and firing on people partying inside.



Nearly two-thirds, or 24 of the people killed were foreign visitors, many from the Middle East, according to Turkey's state-run Anadolu agency. Countries from India to Belgium reported their citizens among the casualties. Close to 70 others were injured.



An estimated 600 people were celebrating inside the club that is often 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.



There was no immediate claim of responsibility for what authorities immediately called a terrorist attack. Turkish officials did not comment on the possible identity or motives of the assailant.



The mass shooting follows more than 30 violent acts that rocked Turkey -- a member of the NATO alliance and a partner in the U.S.-led coalition fighting against the Islamic State (ISIS) group in Syria and Iraq -- in the span of year.



The country endured multiple bombing attacks in 2016, including three in Istanbul alone which authorities blamed on ISIS, a failed coup attempt in July and renewed conflict with Kurdish rebels in the southeast.



President Recep Tayyip Erdogan vehemently condemned "the terror attack in Istanbul's Ortakoy neighborhood in the first hours of 2017" and offered condolences for those who lost their lives, including the "foreign guests."



Among them were an 18-year-old Israeli woman; three Indian citizens; three Lebanese; a woman with dual French-Tunisian citizenship and her Tunisian husband; three Jordanians; and a Belgian national, according to the countries' respective foreign ministries.



Interior Minister Suleyman Soylu said the gunman, who has not been identified, remains at large. "Our security forces have started the necessary operations. God willing, he will be caught in a short period of time," Soylu said.



Private NTV news channel said the assailant entered the upscale nightclub, on the shores of the Bosporus strait, on the European side of the city, dressed in a Santa Claus outfit -- a claim Prime Minister Binali Yildirim denied.



Security camera footage obtained by The Associated Press from Haberturk newspaper shows what appears to be a male assailant dressed in black and carrying a backpack as he shoots down a police officer outside the nightclub.



Footage taken by a different camera inside Reina shows a figure wearing different clothes and what could be a Santa Claus hat. The prime minister denied that the gunman wore a Santa outfit.



Yildirim said the attacker left a gun at the venue and escaped by "taking advantage of the chaos" that ensued. Some customers reportedly jumped into the waters of the Bosporus to escape the attack.



Mehmet Dag, 22, was passing by the club and saw the suspect shoot at a police officer and a bystander. He said the suspect then targeted security guards, gunning them down and entering the club.



"Once he went in, we don't know what happened. There were gun sounds, and after two minutes the sound of an explosion," Dag said.



Turkish media said the local victims include a 22-year-old police officer and a 47-year-old travel agent, both of whom were shot outside the club.



One was given a funeral ceremony Sunday in Istanbul where his two sons joined the mourners gathered around the flag-draped casket, the private Dogan news agency reported.



Ayhan Arik, a tourism company employee who had brought foreign guests to the nightclub, was shot in the head, the news agency said.



The nightclub area remained sealed off on Sunday.



Heavily armed police blocked the snowy street in front of the nightclub where the entrance was covered with blue plastic sheeting below a Turkish flag. Police patrolled the Asian side of the Bosporus on the other side of the club.



Crime scene investigators were seen inside Reina searching through mingled piles of chairs, tables and pieces of clothing left behind during the panic among the guests.



And there were emotional scenes in front of a city morgue where the dead were brought for identification. Some relatives cried out and fell to the ground as they apparently learned the fate of their loved ones.



The U.S. Consulate General in Istanbul on Sunday warned American citizens to keep their movements in the city "to an absolute minimum."



A statement reminded U.S. citizens that extremists "are continuing aggressive efforts to conduct attacks in areas where U.S. citizens and expatriates reside or frequent."



The United States also denied reports in Turkish news outlets and on social media that its security agencies knew in advance that the nightclub in Istanbul was at risk of a terror attack.



The U.S. Embassy in Ankara said in a statement that "contrary to rumors circulating in social media, the U.S. Government had no information about threats to specific entertainment venues, including the Reina Club."



Turkey faces a wide spectrum of security threats.



ISIS claims to have cells in the country. Analysts think it was behind suicide bombings in January and March that targeted tourists on Istanbul's iconic Istiklal Street, as well as a high-casualty suicide bomb and gun attack at Ataturk Airport in June.



In December, ISIS released a video purportedly showing the killing of two Turkish soldiers and urged its supporters to "conquer" Istanbul. Turkey's jets regularly bomb the group in the northern Syrian town of Al-Bab. Turkish authorities have not confirmed the authenticity of the video.



Turkey's violent 2016 also reflects the intensification of an armed conflict between the state and Kurdish rebels. Turkey-based Kurdish groups have claimed multiple suicide attacks. Ankara has said Kurdish affiliates in Syria and Iraq share responsibility.



Complicating matters, the country endured a coup attempt on July 15, blamed by Ankara on a U.S-based Islamist cleric. A state of emergency has been in force since then, and authorities have purged key institutions, including the army and police.



The consecutive calamities have left the nation on edge and kept tourists at bay. In Istanbul, a bustling city bridging Europe and Asia, the toll on the economy is evidenced in the closure of iconic restaurants and lowered hotel prices.



The nightclub attack drew quick condemnation from the West and Russia.



Russian President Vladimir Putin sent his Turkish counterpart a telegram of condolences, saying "it is hard to imagine a more cynical crime than killing innocent people during New Year celebrations."



"However, terrorists don't share moral values. Our common duty is to combat terrorists' aggression," Putin said.



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



Prime Minister Yildirim vowed to keep fighting terror organizations, but noted that, "The terror that happens here today may happen in another country in the world tomorrow."



The Associated Press contributed to this report.