PESHAWAR, Pakistan — Militants attacked a health center used in an ongoing polio campaign in northwest Pakistan on Tuesday, triggering a shootout in which a police officer was killed, local authorities said. Three of the attackers were also killed in the exchange.

No one immediately claimed responsibility for the attack in Orakzai, a district in Khyber Pakhtunkhwa province and a former stronghold of the Pakistani Taliban.

Local police officer Adnan Khan said the attack occurred in the early hours of the morning as health workers were gathering before going on a door-to-door campaign with police escorting polio teams for their safety.

No polio workers were injured in Tuesday’s attack, but another police officer was injured, Khan added.

Also on Tuesday, militants stormed a health center in North Waziristan, another former stronghold of the Pakistani Taliban in Khyber Pakhtunkhwa, wresting weapons from officers and warning health workers who had gathered there not to participate in the anti-polio campaign, said local policeman Shoib. Khan said.

The attackers then left with the confiscated weapons, he said, without giving further details.

Often militants from Pakistan target police and polio workers during campaigns against polio, claiming that vaccination efforts are a Western conspiracy to sterilize children.

Pakistan and Afghanistan are the only two countries in the world where the spread of polio was never stopped. In severe cases, poliomyelitis can cause permanent paralysis and death.

Pakistan launched a new nationwide polio drive on Monday to vaccinate 45 million children under 5 after a surge in new cases. The campaign is the third this year.

Pakistan has recorded 41 cases in 71 districts so far this year, mostly in southwestern Balochistan and southern Sindh provinces, as well as Khyber Pakhtunkhwa and eastern Punjab province.

In Afghanistan, local health authorities said Monday that vaccinations had also begun in 16 of the country’s 34 provinces. The campaign will run for three days and will target 6.2 million children under the age of 5, according to spokesperson Sharafat Zaman.

Data from the World Health Organization says there have been 23 confirmed cases in Afghanistan this year.