At least four people were killed and 16 injured in two car bomb explosions outside a police station in the Shiite holy city of Najaf in south of Baghdad on Monday, a local police source said.

The attack occurred in the morning when a booby-trapped car detonated outside the police station in al-Hussien district in central the city of Najaf, some 160 km south of Baghdad, the source told Xinhua on condition of anonymity.

Afterwards, another car bomb ripped through the police station which is tasked with protecting the highways, the source said.

Many of the victims were policemen, the source added.

Violence and sporadic high profile attacks are still common in the Iraqi cities despite the dramatic decrease of violence over the past few years.