The gunman who killed 39 people in an Istanbul nightclub on New Year's Day in an attack claimed by the Islamic State group appears to have been well versed in guerrilla warfare and may have trained in Syria, a newspaper report and a security source said on Tuesday.

The attacker, who remains at large, shot dead a police officer and a civilian at the entrance to the exclusive Reina nightclub on Sunday. He then opened fire with an automatic rifle inside, reloading his weapon half a dozen times and shooting the wounded as they lay on the ground.

In a statement claiming the attack on Monday, IS said the shooting was revenge for Turkish military involvement in Syria.