KHAR, Pakistan (AP) — Gunmen on motorcycles opened fire Wednesday on police escorting a team of polio workers during a door-to-door vaccination campaign in northwestern Pakistan, killing an officer and a polio worker, police said.

No one immediately claimed responsibility for the attack in Bajur, a district in the Khyber Pakhtunkhwa province and a former stronghold of the Pakistani Taliban, according to local police chief Abdul Aziz.

Pakistan’s Interior Minister Mohsin Naqvi issued a statement condemning the attack.

Pakistan on Monday launched a nationwide polio campaign amid a spike in militant attacks. The potentially fatal, paralyzing disease mostly strikes children under age 5 and typically spreads through contaminated water.

That same day, a roadside bomb hit a vehicle carrying officers assigned to protect health workers conducting polio immunization in the northwestern South Waziristan district, in the same province, wounding six officers and three civilians.

The militant Islamic State group later claimed responsibility for Monday’s attack.

Anti-polio campaigns in Pakistan are regularly marred by violence. Militants target vaccination teams and police assigned to protect them, falsely claiming that the campaigns are a Western conspiracy to sterilize children.

Since January, Pakistan has reported 17 new cases of polio, jeopardizing decades of efforts to eliminate polio in the country. Pakistan and Afghanistan are the only countries in which the spread of polio has never been stopped.