UPDATE: Istanbul governor: at least 35 dead in attack on nightclub

ISTANBUL—At least 35 people were killed in an attack at a nightclub in Istanbul during New Year's celebrations, Istanbul's governor said.

An assailant believed to have been dressed in a Santa Claus costume opened fire at a nightclub in Istanbul during New Year's celebrations, according to local private media.

Governor Vasip Sahin said some 40 other people were wounded in the assault in the early hours of Sunday. Sahin said the incident was a "terror attack" without saying who may have carried it out.

Media reports said more than one assailant may have been involved in the attack and more than 500 people were inside the club at the time.

The attacker is believed to have opened fire at police outside the nightclub before entering and firing on people inside. Some customers jumped into the waters of the Bosporus to escape the attack, the report said.

NTV television said the assailant may still be inside the nightclub.

Footage from the scene 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.

An AP photographer says police cordoned off the area about 2 miles (3 kilometres) away from the nightclub and reported multiple ambulances passing by.

Security measures had been heightened in major Turkish cities, with police barring traffic leading up to key squares in Istanbul and the capital Ankara. In Istanbul, 17,000 police officers were put on duty, some camouflaged as Santa Claus and others as street vendors, state news agency Anadolu reported.

Ankara and Istanbul have been targeted by several attacks in 2016 carried out by the Islamic State group or Kurdish rebels, killing more than 180 people.