ISTANBUL (CBSNEWS)-- An armed assailant has opened fire at a nightclub in Istanbul during New Year’s celebrations, killing at least 35 people and wounding 40, Istanbul’s governor said. 

At least one of the people killed was a police officer, Istanbul governor Vasap Sahin said in a statement. Sahin said the incident was a “terror attack” without saying who may have carried it out.

Private NTV television said a police officer and a civilian were killed in the attack targeting the popular Reina nightclub in Istanbul’s Ortakoy district. 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 Strait 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 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.