Thirty-nine people were killed and 40 wounded in an attack in a nightclub early Sunday as they were celebrating the new year, a report said.

Around 1:15 a.m. Sunday, the gunman shot a police officer who was guarding the front gate at the Reina nightclub, Istanbul Governor Vasip Sahin was quoted as saying in a CNN report.

"He entered the club and attacked innocent people who were there to celebrate the new year. It was a cruel, cold-blooded act," the governor said.

There have been no claims of responsibility. The fate of the attacker was also unclear.

Witnesses in the club said they heard a loud noise, then a security officer told everyone to get out.

One victim said he didn't know how many attackers there were, but he saw one person and hid.

"I got shot in the (expletive) leg, man," he told journalists in English as he was taken into an ambulance. "These crazy people came in shooting everything."

The attacker, who has not yet been identified, opened fire inside the high-end nightclub in the busy Besiktas neighbourhood, according to Turkish state-run news agency Anadolu.

US and Turkish officials called it a terrorist attack, making it the first of 2017.