Police say gunmen have shot dead a police officer protecting a team of polio workers during a U.N.-backed vaccination campaign in northwestern Pakistan.

Police officer Fazal Wahid says no polio workers were wounded in Tuesday's attack in the districts of Mardan in Khyber Pakhtunkhwa province.

No one immediately claimed responsibility for the attack. In December, gunmen killed nine polio workers in similar attacks across Pakistan.

Some militant groups in Pakistan oppose the vaccination campaign, accusing health workers of acting as spies for the United States and claim the polio vaccine is intended to make children sterile.

Pakistan is one of the few remaining places where polio is rampant. As many as 56 polio cases were reported in Pakistan during 2012, down from 190 in 2011.