The police officers were assigned to escort health workers during an anti-polio vaccination campaign in the region.
The bombing in Kolachi, a town in Khyber Pakhtunkhwa Province that borders Afghanistan, was not claimed by any group, but the Taliban mounts regular attacks against polio workers, claiming that vaccination campaigns are a Western conspiracy to sterilize children.
Security forces were searching the area for the attackers, police official Wahid Khan told the media, adding that no polio workers were traveling with the police at the time.
The attack came after Pakistan on February 17 launched a three-day nationwide vaccination campaign against the crippling disease.
Pakistan, Afghanistan, and Nigeria are the only three countries in the world where polio is still endemic.