Istanbul (Turkey) - A gunman has shot his way into an Istanbul nightclub packed with New Year's revellers, killing at least 35 people and wounding more than 40 in what the city's governor described as a terrorist attack. The gunman, believed to have been dressed in a Santa costume, opened fire inside the Reina nightclub in Istanbul's Ortaköy district The assailant shot a police officer and a civilian as he entered the Reina nightclub before opening fire at random inside. The club lies on the shore of the Bosphorus Strait in the Ortakoy district of Turkey's most populous city. US President Barack Obama has expressed his condolences for the attack at a night club in Istanbul and directed his team to offer help to Turkish authorities, the White House said. "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.