ISTANBUL (AP) —Istanbul's governor says an assailant believed to have been dressed in a Santa Claus costume opened fire at a nightclub in Istanbul during New Year's celebrations Saturday, killing at least 39 people and wounding 70 others in what he describes as "a terror attack."

Provincial Gov. Vasip Sahin says the attacker, armed with a long-barreled weapon, killed a policeman and a civilian outside the club before entering and firing on people partying inside.

According to Sahin, the attacker" rained bullets in a very cruel and merciless way on innocent people who were there to celebrate New Year's and have fun."

The whereabouts of the assailant is not known and it's not clear if he's been caught.

Security measures had been heightened in major Turkish cities, with police barring traffic leading up to key squares in Istanbul and the capital, Ankara. In Istanbul, 17,000 police officers were put on duty, some camouflaged as Santa Claus or street vendors.

Ankara and Istanbul have been targeted by several attacks in 2016 carried out by the Islamic State group or Kurdish rebels, killing more than 180 people.

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The White House is condemning what it calls a "horrific terrorist attack" in Istanbul and offering U.S. help to Turkey.

White House spokesman Eric Schultz says President Barack Obama was briefed on the attack by his national security team and asked to be updated as the situation develops. Obama is vacationing in Hawaii this week with his family.

White House National Security Council spokesman Ned Price says the attack on "innocent revelers" celebrating New Year's shows the attackers' savagery. He says the U.S. sends thoughts and prayers to the relatives of those killed.

Price says the U.S. supports its NATO ally Turkey as both countries fight terrorism.

The assailant is believed to have been dressed as Santa Claus when he killed 35 people and wounded 40 more at an Istanbul nightclub.