PESHAWAR, Pakistan (AP) — Gunmen attacked police officers providing security for polio vaccination workers during a door-to-door campaign in northwest Pakistan on Wednesday, killing one of them before fleeing the scene, police said.

The attack occurred in Bannu district in Khyber Pakhtunkhwa province on the third day of a weeklong anti-polio drive to vaccinate 2.7 million children in the province, according to Nasit Shah, a local police official.

No one has claimed responsibility, but Pakistan’s anti-polio campaigns are regularly marked by violence.

Pakistani militants often target polio teams and police assigned to protect them, falsely claiming that vaccination campaigns are a Western conspiracy to sterilize children.

Pakistan and neighboring Afghanistan are the only two remaining countries in the world where polio is endemic, and Pakistan has said it will continue anti-polio campaigns to ensure the eradication of polio.