Suspected militants shot and killed a police officer guarding a team of polio workers in northwestern Pakistan on Tuesday, the latest in a series of attacks on vaccine teams in the country.

Axar.az reports, citing AP, the attack took place in the Matta area of Khyber Pakhtunkhwa province, once a stronghold of the Pakistani Taliban, a day after Pakistan launched a weeklong nationwide vaccination campaign to immunize 45 million children.

Local police official Javed Khan said a team of female polio workers was administering drops to children in a house when “terrorists riding on a motorcycle opened fire" and killed the officer. He said that a search operation was underway to locate and apprehend the assailants.

Prime Minister Shehbaz Sharif condemned the assault in a statement and vowed stern action against those responsible.

No group immediately claimed responsibility for the attack, but suspicion is likely to fall on militants who frequently target health workers and the police officers assigned to protect them.

More than 200 polio workers and police officers protecting them have been killed in Pakistan since the 1990s, according to health and security officials. Militants often falsely claim the vaccination campaigns are part of a Western plot to sterilize Muslim children.

Since January, Pakistan has reported 29 polio cases. During the ongoing campaign, more than 400,000 trained workers are going door-to-door to vaccinate children.

Pakistan and neighboring Afghanistan are the only two countries where transmission of the wild poliovirus has never been stopped, according to the World Health Organization.