At least 35 people have killed and 40 wounded after an assailant opened fire on an Istanbul nightclub Sunday morning.

According to the Associated Press, the attacker, who was reportedly wearing a Santa Claus costume, killed a police officer and a civilian before entering Reina nightclub at around 1:45 a.m. An estimated 600 people were inside the popular venue at the time, celebrating the New Year.

Istanbul Gov. Vasip Sahin called the tragedy a terrorist attack.

“Unfortunately [he] rained bullets in a very cruel and merciless way on innocent people who were there to celebrate New Year's [Eve] and have fun," Sahin said.

The suspect, who has yet to be identified, was reportedly armed with a long-barreled weapon. Other media outlets, including the New York Times, have reported the possibility of multiple shooters.

Law enforcement immediately blocked the surrounding area, while several ambulances arrived on the scene to transport the wounded to hospitals.

Turkey’s major cities have increased security measures, with some 17,000 police officers on duty in Istanbul, state news agency Anadolu reported.

Following the attack, President Barack Obama sent his condolences to Turkey and offered U.S. support.

“This afternoon the president was briefed by his national security team on the attack in Istanbul," White House spokesman Eric Schultz said in a statement. "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."