Three policemen were killed in gunmen attack in north of Baghdad on Tuesday morning, an Interior Ministry source told Xinhua. Unidentified gunmen using weapons fitted with silencers attacked a police checkpoint in Baghdad\'s northeastern district of al-Sha\'ab and killed three policemen before they fled the scene, the source said on condition of anonymity. Iraqi security forces cordoned off the scene and carried out operation in the area, while the police were investigating the incident, the source said. The attack is part of a wave of assassinations by militia and gangs against Iraqi security members, officials and civilians. The attackers\' tactics varied from suicide bombings, gunfire by silenced weapons and sticky bombs attached to vehicles.