PESHAWAR, Pakistan (AP) -- Gunmen riding on a motorcycle shot and killed a police officer protecting polio workers during a U.N.-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 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."

Some Islamic militants oppose the vaccination campaign, accuse health workers of acting as spies for the U.S. and claim the polio vaccine is intended to make Muslim children sterile. Pakistan is one of the few remaining places where polio is still rampant.

In a separate incident in the northwest, a man wounded a polio worker with an axe.

The attacks occurred on the second day of a three-day campaign against polio that was launched by the provincial government. No one claimed responsibility for the shooting in Gullu Dheri, but suspicion fell on militants.

Suspicion of vaccination campaigns heightened considerably after it became known that a Pakistani doctor helped in the U.S. 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 U.S. commandos killed the al-Qaida leader in May 2011.

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

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

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

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

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 paralyze. 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 for vaccination.

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

On Tuesday, gunmen driving in a car opened fire on two police constables who were patrolling on motorcycle in a neighborhood of the provincial capital, Quetta, said Nadir Khan, a local police chief. Both the police officers were killed, he said.

There was no immediate claim of responsibility, but similar attacks against security forces are believed to be the work of Baluch nationalists who have been pushing for a greater say in use of the province's resources.

Also Tuesday, gunmen riding on two motorcycles opened fire on a police station in the country's largest city, Karachi, killing one police constable and injuring two others, said senior police officer Alim Jaffery. He said the gunmen escaped.

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