Dozens of people were reportedly killed early Sunday after an armed suspect entered a nightclub in Istanbul during New Year's celebrations.

Istanbul's governor said at least 35 people had died in the attack, which took place at the popular Reina nightclub in the Ortakoy area, according to the BBC.

At least another 40 people were injured in the attack, with several hundred people reportedly in the nightclub at the time.

Istanbul Governor Vasip Sahin described the event as a terrorist attack, saying the attacker shot a police officer and a civilian as he entered the club before opening fire inside, according to Reuters.

President Obama was briefed on the attack by his national security team while wrapping up his family's annual holiday vacation in Hawaii, a White House official said.

"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 principal deputy press secretary Eric Schultz said in a statement.

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The White House National Security Council also issued a statement condemning the "horrific terrorist attack."

"That such an atrocity could be perpetrated upon innocent revelers, many of whom were celebrating New Year's Eve, underscores the savagery of the attackers," NSC spokesman Ned Price said.

"We offer our thoughts and prayers to the families and loved ones of those killed, and a speedy recovery to the wounded. We reaffirm the support of the United States for Turkey, our NATO ally, in our shared determination to confront and defeat all forms of terrorism."

Turkey's state-run news agency Anadolu said the attacker opened fire in the early morning hours Sunday, but the attacker has not yet been identified. CNN Turkey reported the suspect was allegedly dressed in a Santa costume.

Sahin described the suspect as "a terrorist with a long-range weapon" according to ITV News, while Turkish news channel NTV broadcast images showing dozens of police and emergency services responding to the scene.

State Department deputy spokesman Mark Toner said U.S. officials would remain in close contact with Turkish authorities throughout the investigation, calling the attack "only the latest effort to kill and maim innocent civilians."

"These attacks only reinforce our strong determination to work with the Government of Turkey to counter the scourge of terrorism," he said.

Updated: 9:06 p.m.