Six policemen were killed on Wednesday when armed assailants opened fire at a police van in northwestern Pakistan's restive Khyber Pakhtunkhwa province, police said.

The unknown gunmen targeted the mobile van in Lakki Marwat district, bordering South Waziristan, police said.

The Assistant Sub-Inspector of the police post, Ilum Deen, along with five constables were killed in the attack, police said.

Police contingents rushed to the spot and launched a combing operation to nab the fleeing assailants, District Police Officer Lakki Marwat said.

Pakistan Prime Minister Shahbaz Sharif condemned the incident and Interior Minister Rana Sanaullah sought a report from the Chief Secretary and Police Chief of the province into the attack.

Chief Minister of the province Mehmood Khan also took notice of the incident and sought an immediate report from the Khyber Pakhtunkhwa Police Chief.

Last week two police officials were killed and two others injured when heavily-armed assailants opened fire at the Raghzai police station in the South Waziristan district.