Suspected militants riding a motorcycle shot and killed two police officers assigned to protect polio workers during a door-to-door vaccination campaign Tuesday in northwestern Pakistan, officials said.

The drive-by shooting in Mohmand, a district in Khyber Pakhtunkhwa province that borders Afghanistan, also wounded a third officer.

The police officers were traveling to a nearby village to guard vaccination teams when they came under fire, police officer Ayaz Khan said.