British tourists have been warned to "remain vigilant" in Turkey where at least 35 people were killed when a gunmen believed to have been dressed as Santa Claus opened fire at New Year's revellers.
Some 40 other people in the popular Reina nightclub in Istanbul were wounded in the "terror attack" in the early hours of Sunday, according to the city's governor Vasip Sahin.
The Foreign Commonwealth Office (FCO) said UK tourists should follow the advice of local authorities while remaining vigilant.
Mr Sahin said the assailant, armed with a long-barrelled weapon, killed a policeman and a civilian outside the club in the Ortakoy district before entering and firing at partygoers at around 1.45am local time.
"Unfortunately (he) rained bullets in a very cruel and merciless way on innocent people who were there to celebrate New Year's and have fun," Mr Sahin said.
The FCO was attempting to find out whether any British nationals were injured in the attack at the club, where more than 500 revellers were seeing in 2017.
A spokesman said: "We are in touch with the local authorities following reports of an incident at a night club in Istanbul."
Clubbers reportedly jumped into the waters of the Bosporus strait to escape.
Security in Turkish cities had been heightened with 17,000 police officers, some camouflaged as Santa Claus, on duty in Istanbul, state news agency Anadolu said.
Terrorists from the Islamic State (IS) group and Kurdish rebels targeted Istanbul and Ankara several times inflicting a death toll of more than 180 in 2016.
Turkish-backed rebel groups in Syria have been targeting IS militants.