KHAR, Pakistan (AP) — A roadside bomb exploded Monday near a van carrying police assigned to protect workers in an anti-polio immunization campaign in restive northwestern Pakistan, killing at least six officers and wounding 10 others, officials said.

The attack happened in the former Pakistani Taliban stronghold of Mamund in Khyber Pakhtunkhwa province bordering Afghanistan, police official Kashif Zulfiquar said.

He said some of the wounded officers were in critical condition at a government hospital.

Anti-polio campaigns in Pakistan are regularly marred by violence. Islamic militants often target polio teams and police assigned to protect them, claiming falsely that the vaccination campaigns are a Western conspiracy to sterilize children.

No one immediately claimed responsibility for Monday’s attack, which came hours after authorities launched the first anti-polio campaign of the new year.

Zulfiquar said the campaign has been halted in the area where the attack occurred and all the polio workers are safe. Authorities said it will continue in other parts of the country.

Pakistan and Afghanistan are the only countries in the world where polio remains endemic. Last year, at least six new polio cases were detected in Pakistan, almost all in the northwest where parents often refuse to inoculate children. The outbreak has been a blow to the nation’s efforts to eradicate the disease, which can cause severe paralysis in children.

In 2021, Pakistan reported only one case, raising hopes it was close to eradicating polio. Other cases then began being detected despite anti-polio efforts.

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Associated Press writer Riaz Khan in Peshawar, Pakistan, contributed to this report.