PESHAWAR, Pakistan (AP) -- Gunmen shot and killed a police officer Tuesday who was protecting a team of polio workers during a U.N.-backed vaccination campaign in northwestern Pakistan.
The attack is the latest incident of violence against Pakistan's efforts to eradicate the deadly disease found in only three countries in the world. Militant extremists view the vaccination campaigns as Western-backed plots to gain intelligence in sensitive areas and have frequently targeted the medical staff and those protecting polio teams.
No polio workers were wounded in Tuesday's attack in the Mardan district of Khyber Pakhtunkhwa province, said police officer Fazal Wahid.
At least two attackers were hiding in a field near a narrow road as the polio workers walked by on their way to visit houses in the area, said the Mardan police chief, Inam Jan.
"The polio workers were going door-to-door and one police officer was protecting them when the gunmen suddenly attacked them near an open area and fled," Jan said, adding that the police were searching for the attackers but that so far no one had been arrested.
No one immediately claimed responsibility for the attack and it wasn't immediately known whether the police officer was targeted because he was protecting the polio team or for some other reason.
In 2012, humanitarian workers, including those working to prevent the polio spread, were repeatedly targeted. According to UN figures, 19 humanitarian workers were killed last year in Pakistan. Of those deaths, 11 were related to polio, including a rash of shootings in December when nine polio workers were killed across Pakistan.
In an effort to protect people administering the vaccine, the government has increasingly sent police officers into the field along with the vaccinators only to see them come under attack as well.
On Jan. 29, gunmen riding on a motorcycle shot and killed a police officer protecting polio workers in the Swabi district in Khyber Pakhtunkhwa province.
Some militant groups in Pakistan oppose the vaccination campaign, accusing health workers of acting as spies for the United States or the Pakistani government.
The campaigns are made more complicated by the fact that many Pakistani residents are also suspicious of the repeated vaccination efforts going on across the country and fear the vaccines are intended to make Muslim children sterile.
Pakistan is one of the few remaining countries, along with Afghanistan and Nigeria, where polio is rampant. As many as 56 polio cases were reported in Pakistan during 2012, down from 190 in 2011. Most of the new cases in Pakistan were in the northwest, where the presence of militants makes it difficult to reach children for vaccination.
The virus usually infects children living in unsanitary conditions. It attacks the nerves and can kill or paralyze.
Associated Press writer Rebecca Santana in Islamabad contributed to this report.