PESHAWAR, Pakistan (AP) — Gunmen on motorcycles opened fire on Pakistani police escorting a team of polio workers Tuesday during a door-to-door inoculation campaign in a former Pakistani Taliban stronghold, killing two policemen and a polio worker, police said.

No one immediately claimed responsibility for the attack in North Waziristan, a district in the Khyber Pakhtunkhwa province. The attack came a day after the government launched another anti-polio campaign amid a spike in attacks.

A passerby was also wounded, said Aziz Ullah, a local police official. The attackers fled the scene.

Since April, Pakistan has registered 11 new polio cases — all in North Waziristan, where parents often refuse to inoculate children. The outbreak has been a a blow to the Islamic nation’s efforts to eradicate the disease, which can cause severe paralysis in children.

Pakistan’s anti-polio campaigns are regularly marked by violence. Islamic militants often target polio teams and police assigned to protect them, falsely claiming the vaccination campaigns are a Western conspiracy to sterilize children.

Pakistan and Afghanistan are the only countries in the world where polio remains endemic. In 2021, Pakistan reported only one case, raising hopes it was close to eradicating polio.

North Waziristan was a base for myriad militant groups, including the Pakistani Taliban, until a massive military offensive in recent years claimed to have cleared the region, forcing many militants to flee to Afghanistan. But attacks have been increasing lately, adding to concerns the assaults could further jeopardize anti-polio campaigns.

Before this week, Pakistan's health authorities had carried out three nationwide anti-polio drives this year — in January, March and in May. During the March campaign, gunmen in northwestern Pakistan shot and killed a female polio worker as she was returning home after a day of vaccinations. And in January, gunmen shot and killed a police officer providing security for polio vaccination workers, also in the country’s northwest.