At least 35 people were killed in a gun attack on a popular nightclub in Istanbul, city's Governor Vasip Sahin said on Sunday morning.

Hurriyet Daily News quoted Sahin as saying that the attacker killed a police officer and a civilian at the gate of the nightclub, before entering inside and raving through the crowd celebrating the new year.

At least another 40 were injured in the "terrorist" attack which took place in the Reina nightclub, in the Ortakoy area, at about 1.30 local time (22:30 GMT).

One attacker was involved, the Governor said, while CNN Turk reported he was dressed in a Santa Claus costume.

"A terrorist with a long-range weapon … brutally and savagely carried out this incident by firing bullets on innocent people who were there solely to celebrate the New Year and have fun," BBC quoted Sahin as telling reporters at the scene of the upmarket Reina nightclub, which sits on the banks of Bosphorus in the city's European side.

According to a witness, there were upto 700 people in the nightclub at the time of the attack, some of whom are believed to have jumped into the river to escape.

Some 50 to 60 ambulances were dispatched to the scene.

"The inglorious (attacker) raved through the place with Kalashnikovs. The US intelligence warned over such an attack about one week or 10 days ago and measures have been taken, including the sea front. And look what has happened then," Mehmet Koçarslan told Hürriyet reporter Toygun Atilla adding that such an attack was coming, but could not be prevented.

Turkish television channel NTV said special force police officers were searching the nightclub.

Reina is one of the international night clubs in Istanbul frequented by the high-society, celebs, artists and football stars.

Some 25,000 police officers have been on duty against a possible terror attack in Istanbul during the New Year's Day celebrations in the wake of consecutive bomb attacks across the country in 2016, which claimed hundreds of lives, Hurriyet Daily News added.

Turkey's Radio and Television Supreme Council (RTÜK) also announced a gag order regarding the attack.

President Recep Tayyip Erdogan expressed deep sadness over the attack, Anadolu Agency cited government sources, and received updates from Prime Minister Binali Yildirim and Governor Sahin on the casualties as well as the details of the terrorist act.

Less than a fortnight ago, the Russian ambassador, Andrei Karlov, was shot dead by off-duty Turkish policeman Mevlut Mert Altintas as he gave a speech in the capital Ankara in December.

After the shooting, the killer shouted the murder was in revenge for Russian involvement in the conflict in the Syrian city of Aleppo.

Three weeks ago twin bombings in Istanbul killed at least 45 people, mostly police officers.