Peshawar: Gunmen riding on a motorcycle shot and killed a police officer protecting polio workers during a UN-backed vaccination campaign in northwestern Pakistan on Tuesday, the police said.

The attack took place as dozens of polio workers — including several women — were going door-to-door to vaccinate children in Gullu Dheri village of Swabi district in the Khyber Pakhtunkhwa province, said senior police officer Izhar Shah. None of the polio workers the police officer was protecting were hurt in the attack, he said.

“The polio workers were terrified and immediately went back to their homes after the attack,” Shah told The Associated Press. “The anti-polio drive in that village has been suspended.”

It was the second day of a three-day campaign against polio that was launched by the provincial government. No one claimed responsibility for the killing, but suspicion fell on militants.

Some Islamic militants oppose the vaccination campaign, accuse health workers of acting as spies for the US and claim the polio vaccine is intended to make Muslim children sterile.

Suspicion of vaccination campaigns heightened considerably after it became known that a Pakistani doctor helped in the US hunt for Osama Bin Laden.

The physician, Shakil Afridi, ran a hepatitis vaccination campaign on behalf of the CIA to collect blood samples from Bin Laden’s family at a compound in Abbottabad in Pakistan’s northwest, where US commandos killed the Al Qaida leader in May 2011.

The samples were intended to help the US match the family’s DNA to verify Bin Laden’s presence in the garrison city.

In December, gunmen killed nine polio workers in similar attacks across Pakistan, prompting authorities to suspend the vaccination campaign in the troubled areas. The UN also suspended its field operations in December as a result of the attacks. They have since resumed some field activities, said Michael Coleman, a spokesman with Unicef’s polio campaign.

The latest campaign in Khyber Pakhtunkhwa was launched on Monday to give oral drops to those children who had missed it the first time round.

Pakistan is one of only three countries where the crippling disease is endemic. The virus usually infects children living in “unsanitary conditions”. It attacks the nerves and can kill or paralyse. As many as 56 polio cases were reported in Pakistan during 2012, down from 190 the previous year, according to the United Nations.

Most of the new cases in Pakistan were in the northwest, where the presence of militants makes it difficult to reach children.

Pakistan is also struggling to maintain control of its southern province of Balochistan.