PESHAWAR, Pakistan — Police say gunmen have shot and killed an officer who was escorting a team of polio workers during a vaccination campaign in northwestern Pakistan.

Senior police officer Izhar Shah says none of the workers was harmed in Tuesday's attack in the Swabi district of the Khyber Pakhtunkhwa province.

He says the workers were going door-to-door to vaccinate children against polio when the attack took place. It was the second day of a three-day campaign against polio that was launched by the provincial government.

No one claimed responsibility for the killing.

Gunmen killed nine polio workers in similar attacks across Pakistan in December, prompting authorities to suspend the vaccination campaign in the troubled areas.

Pakistan is one of only three countries where the crippling disease is endemic.