At least 35 people have lost their lives in an attack on a nightclub in Turkey, Istanbul's governor has said.
Among the dead at the Istanbul club is one police officer, Vasip Sahin stated, adding that it was a terror attack.
At least another 40 were injured in the attack which took place in the Reina nightclub, in the Ortakoy area, at about 01:30 local time (22:30 GMT).
The Turkish authorities have imposed a media blackout on coverage of the attack.
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," Mr Sahin told reporters at the scene of the upmarket Reina nightclub, which sits on the banks of Bosphorus in the city's European side.
The club was popular not only with Turks, but with foreigners as well, according to the BBC's Selin Girit in Istanbul.
She said it was unclear what had happened to the gunman at this stage - adding that some eyewitnesses spoke of there being not one but two attackers.
There were reportedly as many as 700 people in the nightclub at the time of the attack, some of whom have jumped into the water to escape.
Dogan news agency reported that some witnesses claimed the attackers were "speaking Arabic" while Turkish television channel NTV said special force police officers were searching the nightclub.
US President Barack Obama, who is on holiday in Hawaii, was among the first international leaders to make a statement after being briefed by his team.
"The president expressed condolences for the innocent lives lost, directed his team to offer appropriate assistance to the Turkish authorities, as necessary, and keep him updated as warranted," White House spokesman Eric Schultz said in a statement.
The Turkish government has ordered a temporary media blackout in the wake of the attack, citing security and public order concerns.
This is not the first time such an order has been made.
Istanbul was already on high alert with some 17,000 police officers on duty in the city, following a string of terror attacks in recent months.