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The attack took place in Lakki Marwat district, part of the troubled Khyber Pakhtunkhwa province that borders Afghanistan.
Police said eight to 10 Taliban fighters ambushed a police vehicle, first by hurling a grenade and then opening fire with Kalashnikovs after policemen tried to retaliate.
"Three policemen were killed and one was injured. Taliban later escaped," Mohammad Gulzar, the district police chief, told AFP.
Gulzar said Taliban were hiding on both sides of the road and escaped in two vehicles after the attack.