Islamabad - Gunmen in northwestern Pakistan killed three members of a polio vaccination team and injured another Tuesday, even as the country experiences a fresh outbreak of the infectious, crippling disease.

Officials said the early morning shooting incident occurred during a door-to-door immunization drive in North Waziristan, a turbulent district on the border with Afghanistan. It left a health worker and two policemen escorting the polio team dead.

Wild polio virus has paralyzed eleven children in Pakistan this year, the largest number in the world, and all of the cases are from the Waziristan district, according to the national eradication program.

No group immediately took responsibility for Tuesday's deadly attack.

Islamist insurgents are active in North Waziristan and oppose anti-polio drives as a government effort to collect intelligence on their hideouts.

Afghanistan and Pakistan are the only two countries where polio continues to paralyze children, although case numbers in recent years have significantly declined on both sides of the border.

Afghan officials have reported one case of wild polio virus in 2022 and four in 2021.

Pakistan reported its first polio case of 2022 in early April after having gone nearly 15 months without a single child being crippled by the virus.

Polio paralyzed about 20,000 Pakistani children a year in the early 1990s.

The current immunization campaign in Pakistan started on Monday, covering 25 "very high-risk districts for polio" across the country, with a goal to vaccinate more than 12 million children.

"I particularly urge all parents and caregivers to get their children vaccinated instead of hiding them or refusing to take the necessary drops during all vaccination drives," Shahzad Baig, the coordinator of the national emergency operation center, said in a pre-campaign statement.

Pakistani health officials say that many parents in North Waziristan and surrounding districts continue to refuse polio drops for their children during national vaccination campaigns while others resent repeated door-to-door visits by vaccinators as intrusive.

The refusals stem from suspicions the immunization is a Western-led conspiracy to sterilize Muslim children. The false information has also triggered attacks against health care workers and security forces escorting them, resulting in the deaths of scores of people in recent years and slowing down the eradication efforts.

FILE - Relatives sit beside the body of what they say is a female polio vaccinator who was killed by unidentified gunman, as they protest in front of the district commissioner office in Chaman, Pakistan, Apr. 25, 2019.

Pakistani officials have documented incidents in which parents suspicious of inoculation campaigns sometimes in collusion with health workers get hold off special markers used by vaccinators to put a colored spot on the fingers of children who have been vaccinated. The finger marking is used to determine the exact scale of refusal rates.

Health experts say polio mainly affects children under the age of five years and invades their nervous system, causing paralysis or even death.