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, 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 in the Khyber Pakhtunkhwa province, a mountainous region near the Afghanistan border.

None of the polio workers the police officer was protecting were hurt in the attack, said senior police officer Izhar Shah

"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 in the area have previously accused health workers of acting as spies for the U.S. and claim the polio vaccine is intended to make Muslim children sterile.

Bin Laden’s killing heightens suspicion

Militant suspicion of vaccination campaigns was racheted up after it was revealed that a Pakistani doctor helped the U.S. hunt for Osama bin Laden.

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 U.S. commandos later killed the al-Qaeda leader in May 2011.

The samples were intended to help the CIA match the family's DNA to verify bin Laden's presence in the garrison city.

In Zero Dark Thirty, the Oscar-nominated film about the search for bin Laden, a short scene shows a man going to the compound where the terrorist leader was hiding as part of a vaccination campaign. But in the movie, it's portrayed as an anti-polio campaign instead of anti-hepatitis.

Attacks stall vaccination work

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A Pakistani anti-polio worker condemns violence against vaccination workers in Karachi, after nine polio workers were killed by militants in Dec. 2012. (Akhtar Soomro/Reuters)

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, though UNICEF's polio campaign’s spokesperson Michael Coleman says some work has started again.   The latest campaign in Khyber Pakhtunkhwa was launched Monday to give oral vaccination drops to those children who had missed it the first time round.

Pakistan is one of only three countries where the crippling disease is still endemic. The virus, which attacks the nerves and can kill or paralyze, usually infects children living in unsanitary conditions.

As many as 56 polio cases were reported in Pakistan during 2012, down from 190 the previous year, according to the UN. Major organizations like Rotary International and the Bill and Melinda Gates Foundation are helping efforts to eradicate the disease, which is also found in northern Nigeria, Afganistan and India.