Turkish police struggled to track down a gunman who attacked New Year's Eve revelers at a popular Istanbul nightclub, killing at least 39 people, including two Indians.

Close to 70 more were wounded.

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The attacker, armed with a long-barreled weapon, killed a policeman and a civilian outside the Reina club around 1:15 am before entering and firing at people partying inside, Istanbul Governor Vasip Sahin said.

"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.

Nearly two-thirds of the people killed were foreigners, many from the Middle East, Turkey's state-run Anadolu Agency said. It said the bodies of 25 foreign nationals killed in the attack would be delivered to their families on Monday.

Countries from India to Belgium reported their citizens among the casualties.

An estimated 600 people were celebrating inside the club, which is often frequented by famous locals, including singers, actors and sports stars. Several shocked revelers were seen fleeing the scene after the shooting 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 gunman.

The mass shooting followed more than 30 violent acts over the past year in Turkey, which is a member of the NATO alliance and a partner in the US-led coalition fighting against the Islamic State group in Syria and Iraq.

The country endured multiple bombings in 2016, including three in Istanbul alone that authorities blamed on IS, 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 the dead were an 18-year-old Israeli woman, three Indians, three Lebanese, a woman with dual French-Tunisian citizenship and her Tunisian husband, three Jordanians, a Belgian national, a Kuwaiti citizen and a Canadian, according to those countries' governments and a diplomat.

A US State Department official, who spoke only on condition of anonymity, said one American man was among those wounded.

Turkey's minister for family and social policies, Fatma Betul Sayan Kaya, said citizens of Saudi Arabia, Morocco, Lebanon and Libya were among those injured.

Interior Minister Suleyman Soylu said the gunman, who had not been identified, remained at large.