The gunmen opened fire as the policemen were heading on foot to the town of Lower Dir in the Khyber Pakhtunkhwa province, bordering Afghanistan, said local police official Saeedur Rehman. He said the attackers fled the scene and a search was underway to find them.

The policemen were to escort a team of medics going house-to-house to vaccinate children against the crippling disease. No one immediately claimed responsibility for the shooting.

Pakistan regularly carries out anti-polio drives, despite threats from the Taliban who claim the campaign is a Western conspiracy to sterilize children. Polio teams and security forces escorting them are often targeted in deadly attacks.

The latest, three-day anti-polio campaign started Monday. Rehman said that despite Wednesday's attack, the teams would carry on administering polio drops to children in Lower Dir till the end of the day.

Pakistan is one of three countries in the world where polio is still endemic; the other two are Afghanistan and Nigeria . So far this year, 94 new polio cases have been reported in Pakistan, compared to only 12 in 2018.

Authorities say the increase in polio cases was mainly because of the parents' refusal to let their children be vaccinated. Prime Minister Imran Khan's government has deployed additional police forces to guard polio teams and has appealed, with the support of clerics, on the parents to allow their children to get polio drops.

The latest anti-polio campaign was to include 39.5 million children up to five years of age across the entire country. Pakistan has an estimated population of 220 million.