PESHAWAR, Pakistan (AP) — A roadside bombing targeted a police vehicle in northwestern Pakistan on Tuesday, killing a policeman and wounding three others, officials said. The police were assigned to escort health workers during an anti-polio vaccination campaign in the region.

No one immediately claimed responsibility for the bombing in Kolachi, a town in the Khyber Pakhtunkhwa province which borders Afghanistan. Pakistan regularly carries out anti-polio drives, despite attacks and threats by the Taliban who claim the campaign is a Western conspiracy to sterilize children.

Pakistani security forces were searching the area for the attackers, said police official Wahid Khan. No polio workers were travelling with the police at the time of the bombing, he added.

Attacks on anti-polio campaigns increased in the years following revelations that a fake hepatitis vaccination campaign was used as a ruse by the CIA in the hunt for al-Qaida leader Osama bin Laden, who was killed by U.S. commandos in 2011 in Pakistan.

The latest attack came after Pakistan on Monday launched a three-day nationwide vaccination campaign against the crippling disease.

Pakistan, Afghanistan and Nigeria are the only three countries in the world where polio is still endemic. Pakistan had hoped to make the country polio-free in 2018 but failed to meet the target because of a sudden surge in the new polio cases. Since January, 17 new cases of polio have been reported in Pakistan.