A Pakistani woman holds her child to be vaccinated by a health worker against polio.
Gunmen shot dead a policeman and wounded another on Friday as they headed to Pakistan's northwestern town of Topi to guard an ongoing polio campaign, officials said.
It was the latest in a series of attacks by militants targeting polio teams following the imposition of an official ban by the Taliban last year, who see inoculation campaigns as a cover for espionage.
The two policemen were riding a motorbike and were intercepted and attacked by at least four gunmen as they left Swabi town for Topi.
"The gunmen opened fire on the policemen, killing one of them on the spot and critically wounding the other," senior local police official Muhammad Sajjad Khan told AFP.
He said the pair had been deputed on security duty for an ongoing polio vaccination campaign the area.
An intelligence official also confirmed the incident, which brings the toll to at least 26 deaths since the June 2012 ban.
The attack comes despite a recent fatwa by a prominent Pakistani religious scholar known as the "Father of the Taliban", who urged parents to immunise their children against polio and other life-threatening diseases and said vaccinations were compliant with Sharia.
Pakistan is one of only three countries in the world where polio is still endemic, but efforts to stamp out the crippling disease have been hit by repeated attacks on health teams.
Officials blame the violence and suspicions about the vaccine for a surge in cases. According to the World Health Organisation, Pakistan recorded 72 cases of polio this year compared to 58 in 2012.
New Delhi on Wednesday announced it would require citizens from Pakistan and other polio-affected nations travelling to India to take a mandatory polio vaccination at least six weeks prior to their departure.