Dozens of people have been killed as they celebrated New Year at a Turkish nightclub in a suspected terror attack.

Officials said at least one gunman reportedly wearing a Santa costume murdered a police officer and a civilian outside Istanbul’s famed Reina nightclub before bursting inside.

The attacker opened fire into the crowd of up to 700 people, killing at least 39 – including 15 foreigners – and injuring dozens more in the Ortakoy district.

Interior minister Suleyman Soylu said the attacker was still at large and 69 people were being treated in hospital following the shooting.

"Efforts to find the terrorist are continuing," he said. "Our security forces have started the necessary operations. God willing he will be caught in a short period of time."

Medics carry a wounded person after an attack at a popular nightclub in Istanbul on 1 January (AP)

Local reports said some people jumped into the waters of the Bosphorus strait to escape the massacre, which started shortly before 1.30am local time (11.30pm GMT).

CCTV footage from inside the club was broadcast by Turkish media, appearing to show a gunman dressed as Father Christmas walking through abandoned drinks booths.

Witnesses said the man was wearing a Santa costume and speaking Arabic, suggesting he was not Turkish.

One survivor said she had to climb out from beneath bodies inside the nightclub to flee the attack, where her husband was injured.

“Before I could understand what was happening, my husband fell on top me,” Sinem Uyanik told the Associated Press outside Istanbul's Sisli Hospital. “I had to lift several bodies from on top of me before I could get out.”

Vasip Sahin, the governor of Istanbul, described the event as a “terror attack” but did not name who he thought responsible.

“The attacker - in the most brutal and merciless way - targeted innocent people who had only come here to celebrate the New Year and have fun,” Mr Sahin said.

In pictures: Istanbul nightclub attacked during New Year party Show all 9 1 /9 In pictures: Istanbul nightclub attacked during New Year party In pictures: Istanbul nightclub attacked during New Year party People talk to medics in an ambulance near the scene of an attack in Istanbul on 1 January 2017 AP In pictures: Istanbul nightclub attacked during New Year party People near the scene of an attack in Istanbul on 1 January 2017 AP In pictures: Istanbul nightclub attacked during New Year party Police secure area near an Istanbul nightclub, Turkey, January 1, 2017. Reuters In pictures: Istanbul nightclub attacked during New Year party Ambulances line up on a road leading to a nightclub where a gun attack took place during a New Year party in Istanbul, Turkey Reuters In pictures: Istanbul nightclub attacked during New Year party Turkish special force police officers and ambulances are seen at the site of an armed attack January 1, 2017 in Istanbul Getty In pictures: Istanbul nightclub attacked during New Year party People embrace near the scene of an attack in Istanbul, on New Year's Day AP In pictures: Istanbul nightclub attacked during New Year party Medics carry a wounded person after an attack at a popular nightclub in Istanbul on 1 January AP In pictures: Istanbul nightclub attacked during New Year party Turkish police secure the area at Ortakoy district under Bosphorus Bridge after the attack on Reina nightclub Getty Images In pictures: Istanbul nightclub attacked during New Year party People leave a nightclub in Istanbul after it was attacked on 1 January AP

Amid the manhunt, armed police blocked off the area as the roads were lined with dozens of ambulances and clubbers wearing suits and cocktail dresses poured into the street.

The Turkish government imposed a temporary media blackout on local coverage of the attack, banning the publication or broadcast of anything that could cause “fear in the public, panic and disorder and which may serve the aims of terrorist organisations”.

As with those imposed after previous attacks, the order covered the “moment of attack; aftermath of the attack and site of crime; public servants conducting their jobs; injured and dead, exaggerated analysis and anything related to the people suspected of involvement with the attack, their methods of transport and whereabouts”.

President Recep Tayyip Erdoğan was reportedly being updated on the investigation, while his justice minister Bekir Bozdağ vowed that “no terror attack will destroy our unity”.

Barack Obama was among the world leaders offering assistance to Turkey following the attack – one of a string of recent massacres by Isis and Kurdish extremist groups.

“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.

Turkish special force police officers and ambulances are seen at the site of an armed attack January 1, 2017 in Istanbul (Getty)

Nato Secretary-General Jens Stoltenberg and Federica Mogherini, the EU representative for foreign affairs, also expressed their condolences.

“2017 starts with an attack in Istanbul,” Ms Mogherini wrote on Twitter. “Our thoughts are with victims and their loved ones. We continue to work to prevent these tragedies.”

Germany’s foreign ministry said it was “distraught and mourn with the people of Istanbul”.

The attack sparked warnings for British tourists and residents in the Turkish city.

Travel advice issued by the Foreign and Commonwealth Office urged people to “remain vigilant” as it worked to find out whether any British citizens were affected.

"We are in touch with the local authorities following reports of an incident at a night club in Istanbul,” a spokesperson said.

Security in Turkish cities had been heightened for New Year’s Eve, with 17,000 police officers, including some camouflaged as Santa Claus, on duty in Istanbul, according to the state-run Anadolu news agency.

The latest attack came at the end of a bloody year in Turkey, where terror attacks by Isis and Kurdish extremist groups inflicted a death toll of more than 180 in 2016.

38 killed in twin bombings outside Istanbul football stadium

Twin bombings carried out by the Kurdistan Freedom Falcons (TAK) killed 44 people on 10 December, while other blasts have targeted police and the security services.

Isis was believed to be behind the attack on Istanbul Airport in July, as well as previous suicide bombings targeting tourists and shoppers.

The group has only claimed responsibility for targeted attacks against activists, Americans and security forces in Turkey rather than indiscriminate massacres – a technique analysts believe aims to avoid alienating the Sunni Muslim majority country.

Recent propaganda and statements by leader Abu Bakr al-Baghdadi and new spokesman Abu Hasan al-Muhajir have specifically called for attacks in Turkey, which is backing rebel groups attacking its territories in Syria.

Night clubs and drinking venues have previously been targeted by supporters of the group, which classes anyone drinking alcohol as “disbelievers”, such as in the Paris attacks.