At least 35 people have lost their lives in an attack on a nightclub in Istanbul, the city's governor has said.

Among the dead 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).

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.

There were reportedly as many as 700 people in the nightclub at the time of the attack, some of whom are believed to have jumped into the river 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.

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.

Many were carried out by so-called Islamic State (IS) or Kurdish militants.

There were reportedly as many as 700 people in the nightclub at the time of the attack, some of whom are believed to have jumped into the river 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.

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.

Many were carried out by so-called Islamic State (IS) or Kurdish militants.

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.

10 December: Twin bomb attack outside a football stadium in Istanbul kills 44 people, Kurdish militant group claims responsibility

20 August: Bomb attack on wedding party in Gaziantep kills at least 30 people, IS suspected

30 July: 35 Kurdish fighters try to storm a military base and are killed by the Turkish army

28 June: A gun and bomb attack on Ataturk airport in Istanbul kills 41 people, in an attack blamed on IS militants

13 March: 37 people are killed by Kurdish militants in a suicide car bombing in Ankara

17 February: 28 people die in an attack on a military convoy in Ankara